
QmAMSm 
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COPYRIGHT DEPOSIT 



RESEARCH METHODS 



IN 



ECOLOGY 



BY 

FREDERIC EDWARD CLEMENTS, Ph.D. 

ASSOCIATE PROFESSOR OF PLANT PHYSIOLOGY 
IN THE UNIVERSITY OF NEBRASKA 



ILLUSTRATED 



t> » • 

i > 



LINCOLN, NEBRASKA 

The University Publishing Company 

1905 



.C62 



LIBRARY ot 30NGRESS 
Two Copies rtewaveu 

jun a? iyu5 

GLASS <*- AAC Not 
COPY 6. 



Copyright, 1905 
By FREDERIC E. CLEMENTS and IRVING S. CUTTER 



All rights reserved 



press of 5acob IRortb 8. Company 

Lincoln, Nebraska 



PREFACE 

The present volume is intended as a handbook for investigators and for 
advanced students of ecology, and not as a text-book of the subject. An 
elementary text-book covering the same field, but adapted to the needs of 
undergraduate students, is in preparation. The handbook is essentially an 
account of the methods used by the author in his studies of the last eight 
years, during which a serious attempt has been made to discover and to 
correlate the fundamental points of view in the vast field of vegetation. 
No endeavor is made to treat any portion of the subject exhaustively, since 
a discussion of general methods and general principles is of much greater 
value in the present condition of ecology. The somewhat unequal treat- 
ment given the different subjects is due to the fact that it has been found 
possible to develop some of these more rapidly than others. Finally, it must 
be constantly kept in mind that ecology is still in a very plastic condition, 
and in consequence, methods, fundamental principles, and matters of nomen- 
clature and terminology must be approached without prejudice in order that 
the best possible development of this field may be attained. 

Grateful acknowledgment for criticisms and suggestions is made to Pro- 
fessor Doctor Charles E. Bessey and Professor Doctor Roscoe Pound, who 
have read the text. The author is under especial obligations to Doctor Edith 
S. Clements for the drawings of leaf types, as well as for reading and crit- 
icising the manuscript. Professor Goodwin D. Swezey, Professor of As- 
tronomy in the University of Nebraska, has kindly furnished much material 
for the determination of the sun's altitude, and consequent light intensities, 
and has read the section devoted to light. Mr. George A. Loveland, Di- 
rector of the Nebraska Section of the U. S. Weather Bureau, has contributed 
many helpful suggestions to the discussion of meteorological instruments. 
To Nella Schlesinger, Alice Venters, and George L. Fawcett, advanced stu- 
dents in experimental ecology, the author is indebted for many experiments 
which have been used in the discussion of adjustment and adaptation. 

Acknowledgment is also made to the following for various cuts : Henry 
J. Green, Brooklyn, New York ; Julien P. Friez, Baltimore, Maryland ; C. H. 
Stocking Co., Chicago, Illinois ; Draper Manufacturing Co., New York city ; 
Gundlach-Manhattan Optical Co., Rochester, New York; Rochester Optical 
Co., Rochester, New York; Bausch and Lomb Optical Co., Rochester, New 
York. 

FREDERIC EDWARD CLEMENTS. 
The University of Nebraska, 
May, 1905. 



CONTENTS 



Chapter I. The Foundation of Ecology 



THE NEED OF A SYSTEM 



1. The scope of ecology 

2. Ecology and physiology 
Historical Development 

3. Geographical distribution 

4. The plant formation 

5. Plant succession 

6. Ecological phytogeography 

7. Experimental ecology 



Ecology of the habitat 



IT. 

12. 

13- 

14. 

15- 



9. The evidence from historical development 
Present Status of Ecology 

10. The lack of special training 
Descriptive ecology 
The value of floristic 
Reconnaissance and investigation 
Resident investigation 
The dangers of a restricted field 
Applications of Ecology 

16. The subjects touched by ecology 

17. Physiology and pathology 

18. Experimental evolution 

19. Taxonomy .... 

20. Forestry 
Physiography 
Soil physics 
Zoogeography 
Sociology 



2T. 
22. 

23. 

24. 



THE ESSENTIALS OF A SYSTEM 



25. Cause and effect: habitat and plant 

26. The place of function 



PAGE 

1 

1 

2 

2 

3 

4 

4 

5 
6 

6 

7 
8 

8 

9 
9 

10 
11 
11 
12 

14 

15 

15 

15 
16 



16 
17 



VI 



CONTENTS 



Chapter II. The Habitat 



CONCEPT AND ANALYSIS 

2.J. Definition of the habitat 

28. Factors 

Classification of Factors 

29. The nature of factors 

30. The influence of factors 
Determination of Factors 

31. The need of exact measurement 

32. The value of meteorological methods 

33. Habitat determination 

34. Determinable and efficient differences 
Instrumentation 

35. Methods 

36. Method of simple instruments 

37. Method of automatic instruments 

38. Combined methods .... 



PAGE 

18 
18 

19 
19 

20 
20 
21 
21 

22 
22 

23 
23 



CONSTRUCTION AND USE OF INSTRUMENTS 



39. The selection of instruments 
Water-content 

40. Value of different instruments . 
Geotome methods 

41. The geotome .... 

42. Soil borers .... 
Taking samples of soil 
Weighing 
Computation .... 

46. Time and location of readings 

47. Location of readings 
Depth of samples . . 
Check and control instruments 

Physical and Physiological Water 
50. The availability of soil water 
Terms ..... 

Chresard determination under control 
Chresard readings in the field . 
Chresard values of different soils 



43- 
44- 
45- 



48. 
49- 



5*. 

52. 

53- 
54- 



24 

25 

25 
26 

26 

27 

28 

28 

29 
30 
30 

30 
3i 
32 
33 
34 



CONTENTS 



Vll 



Records and Results 

55. The field record 

56. The permanent record 

57. Sums and means 

58. Curves 
Humidity 

59. Instruments 
Psychrometers 

60. Kinds 

61. The sling psychrometer 

62. Readings 

63. Cog psychrometer 

64. Construction and use 

65. Hygrometers 
Psychrographs 

66. The Draper psychrograph 

67. Placing the instrument 

68. Regulating and operating the instrument 

69. The weekly visit 
Humidity Readings and Records 

70. The time of readings 

71. Place and height 

72. Check instruments 
7^. Humidity tables 

74. Sums, means, and curves 

Conversion scale for temperatures 

75. Records 



PAGE 



Light 



76. Methods .... 
The Photometer 

77. Construction 

78. Filling the photometer 

79. Making readings 

80. The Dawson-Lander sun recorder 

81. The selagraph 
Standards 

82. Use ..... 

83. Making a standard 

84. Kinds of standards 
Readings 

85. Time 

Chart for determining sun's altitude 



35 
36 
36 
37 



37 

37 
38 
39 

39 
40 

40 

4i 

42 

43 
44 

44 

45 

45 
46 

47 
48 

48 

49 
50 
50 
5i 

52 

53 
53 
54 

55 

0/ 



Vlll 



CONTENTS 



86. Table for determining apparent noon 

87. Place 

Table of intensity at various angles 
Reflected and Absorbed Light • 

88. The fate of incident light 

89. Methods of determination 

90. Leaf and epidermis prints 
Expression of Results 

91. Light records .... 

92. Light sums, means and curves 



Temperature 



95- 
96. 

97- 
98. 



Thermometers 

94. Air thermometers 
Soil thermometers 
Maximum-minimum thermometers 
Radiation thermometers 
Thermographs 
Readings 

99. Time . . . 

100. Place and height 
Expression of Results 

101. Temperature records 

102. Temperature sums and means 

103. Temperature curves 

104. Plant temperatures 

Precipitation 

105. General relations 

106. Tile rain gauge 

107. Precipitation records . ' 



Wind 



108. Value of readings 

109. The anemometer 
no. Records 



Soil 



in. 
112. 

"3- 

114. 

11=;. 



Soil as a factor 
The value of soil surveys 
The origin of soils 
The structure of soils 
Mechanical analvsis 



PAGE 

58 

59 
60 

60 
61 
62 

63 
63 

64 

64 
64 

65 
67 

67 

69 
70 

70 
70 

7i 
7i 

72 

73 
74 

74 

75 
76 

76 

77 
77 
78 
79 



CONTENTS 



IX 






116. Kinds of soils 

117. The chemical nature of soils 
Physiography 

118. Factors . . 
Altitude 

119. Analysis into factors 

120. The barometer 
Slope 

121. Concept 

122. The clinometer 

123. The trechometer 
Exposure 

124. Exposure 

125. Surface 

126. Record of physiographic factors 

127. Topography 
Biotic Factors 

128. Influence and importance 

129. Animals 

130. Plants 



PAGE 

79 
80 

80 

81 

82 

83 
83 
84 

85 

85 
86 

86 

86 

87 
87 



METHODS OF HABITAT INVESTIGATION 



131. 

Method of 
132. 

133. 

134- 

1 35- 
Method of 

136. 

Expression 

137- 

Factor 
138. 

Factor 

139- 

140. 

141. 

142. 
Factor 

143. 



Simple Instruments 

Choice of stations 

Time of readings 

Details of the method . 

Records 
Ecograph Batteries 

of Physical Factor Results 
The form of results 
Records 



Curves 

Plotting 

Kinds of curves . 

Combinations of curves 

The amplitude of curves 

Means and Sums 



88 

88 

89 
89 
9 1 

92 

94 

94 

95 
96 

96 

98 
98 



CONTENTS 



Chapter III. The Plant 



STIMULUS AND RESPONSE 

General Relations 

144. The nature of stimuli 

145. The kinds of stimuli . 

146. The nature of response 

147. Adjustment and adaptation 

148. The measurement of response 

149. Plasticity and fixity 

150. The law of extremes . 

151. The method of working hypotheses 
Hydroharmose 

Adjustment 

152. Water as a stimulus 

153. The influence of other factors upon water . 

154. Response 

155. The measurement of absorption . 

156. The quantitative relation of absorption and transpiration 

157. Measurement of transpiration 

158. Field methods ...... 

159. Expression of results .... 

160. Coefficient of transpiration 
Adaptation 

161. Modifications due to water stimuli 

162. Modifications due to a small water supply 

163. The decrease of water loss . 

164. The increase of water supply 

165. Modifications due to an excessive water supply 

166. Plant types 

167. Xerophytic types 

168. Types of leaf xerophytes . 

169. Types of stem xerophytes . 

170. Bog plants .... 

171. Hydrophytic types 
Photoharmose 

Adjustment 

172. Light as a stimulus 

173. The reception of light stimuli 

174. The response of the chloroplast 

175. Aeration and translocation . 



PAGE 

100 
100 

IOI 
102 
103 
IO4 

105 
I06 



I07 
I07 
I08 
IO9 
III 

113 
114 

Il6 

117 

Il8 
Il8 
Il8 
121 
121 
122 
122 
123 

125 
126 
127 



I29 

131 
132 

134 



CONTENTS 



XI 



176. The measurement of responses to light 
Adaptation 

177. Influence of chloroplasts upon form and structure 

178. Form of leaves and stems 

179. Modification of the epidermis .... 

180. The differentiation of the chlorenchym , 

181. Types of leaves ....... 

182. Heliophytes and sciophytes .... 



PAGE 

135 

138 
139 

140 
142 

144 



EXPERIMENTAL EVOLUTION 



183. 
184. 

185. 
186. 

187. 

Method of 

188. 

189. 

190. 
Method of 

191. 

192. 

193- 

194. 

Method of 

195. 
196. 

197. 



Scope . . . 

Fundamental lines of inquiry 

Ancestral form and structure 

Variation and mutation 

Methods 
Natural Experiment 

Selection of species . 

Determination of factors 

Method of record 
Habitat Cultures 

Scope and advantages 

Methods 

Transfer . 

Modification of the habitat 
Control Cultures 

Scope and procedure . 

Water-content series . 

Light series 



145 
146 

146 

147 
149 

149 

151 

152 

153 
153 
154 
156 

157 
158 
160 



Chapter IV. The Formation 



METHODS OF INVESTIGATION AND RECORD 



198. The need of exact methods . 
Quadrats 

199. Uses ...... 

200. Possible objections 
Kinds of Quadrats and Their Use 

201. Size and kinds . 

202. Tapes and stakes 

203. Locating quadrats 



161 

161 
163 

164 
164 

165 



Xll 



CONTENTS 



The List Quadrat 

204. Description 

205. Manner of use . 

206. Table of abundance . 
The Chart Quadrat 

207. Description and use . 

208. The chart . 

209. Mapping 

210. Factors and photographs 
The Permanent Quadrat 

211. Description and uses . 

212. Manner of use . 
The Denuded Quadrat 

213. Description 

214. Methods of denuding and recording 

215. Physical factors . 
Aquatic Quadrats 

216. Scope . . 

Transects 

217. The transect 
The Line Transect 

218. Description and method 

219. The location and size . 
The Belt Transect 

220. Details .... 
The Permanent Transect 

221. Advantages 

222. Details . 
The Denuded Transect 

223 

The Layer Transect 

224 

Ecotone Charts 

225. ... 

The Migration Circle 

226. Purpose 

227. Location and method . 

228. The denuded circle 

229. Photographs 

Cartography 

230. Value of cartographic methods 



PAGE 

165 

166 
166 

167 
168 
168 
170 

170 
172 

173 
174 
175 

175 



176 



176 
177 

178 

179 
179 

180 
180 

181 

182 
182 

183 

183 

. 183 



CONTENTS 



Xlll 



231. Standard scale 

232. Color scheme 

233. Formation and vegetation maps 

234. Continental maps 
Photography 

"235 

236. The camera and its accessories 

237. The choice of a camera 

238. The use of the camera 

239. The sequence of details 

240. The time of exposure . 

241. Developing .... 

242. Finishing 
Formation and Succession Herbaria 

243. Concept and purpose . 

244. Details of collecting 

245. Arrangement 

2a6. Succession herbaria . 



PAGE 

184 
184 
185 
187 

188 
188 
190 
191 
192 

193 
194 

195 

196 
197 
197 
198 



DEVELOPMENT AND STRUCTURE 



247. Vegetation an organism 

248. Vegetation essentially dynamic 

249. Functions and structures . 
Association 

250. Concept .... 

251. Causes .... 

252. Aggregation 
Kinds of Association 

253. Categories .... 

254. Stratum association 

255. Ground association 

256. Species guild association 

257. Light association 

258. Water-content association . 



THE DEVELOPMENT OF THE FORMATION 



p 2 59- • 
Invasion 

260. 

Migration 

261. 



199 
199 
199 

200 
201 
203 

204 
204 
205 
206 
206 
208 

210 
210 
210 



XIV 



CONTENTS 



262. Mobility • . 

263. Organs for dissemination 

264. Contrivances for dissemination 

265. Position of disseminules 
. 266. Seed production . 

267. Agents of migration . 

268. The direction of migration 
Ecesis 

269. Concept 

270. Germination of the seed 

271. Adjustment to the habitat 
Barriers 

2J2. Concept 

273. Physical barriers 

274. Biological barriers 

275. Influence of barriers . 
Endemism 

276. Concept 

277. Causes 

278. Significance 
Polyphylesis and Polygenesis 

279. Concept 

280. Proofs of polygenesis 

281. Origin by polyphylesis 
Kinds of Invasion 

282. Continuous and intermittent invasion 

283. Complete and partial invasion 

284. Permanent and temporary invasion 
Manner of Invasion 

285. Entrance into the habitat 

286. Influence of levels 
Investigation of Invasion 

287 

Succession 

288. Concept .... 

289. Kinds of succession . 
Primary Successions 

290 

29 1. Succession through elevation 

292. Succession through volcanic action 

293. Weathering .... 



PAGE 

211 
21 £ 
212 
214 

215 
216 

219 

220 
221 
223 

224 

225 
225 
226 

227 
228 
228 

230 
231 
232 

234 
235 
235 

236 
238 

239 

239 
240 

241 
241 
242 

243 



CONTENTS 



XV 



300. 
301. 
302. 

303- 

304. 

305. 
306. 

307- 



310. 

3"- 
312. 

313. 

314. 



294. Succession in residuary soils 

295. Succession in colluvial soils 

296. Succession in alluvial soils 

297. Succession in aeolian soils 

298. Succession in glacial soils 
Secondary Successions 

299. . 
Succession in eroded soils 
Succession in flooded soils 
Succession by subsidence 
Succession in land slips 
Succession in drained or dried soils 
Succession by animal agency 
Succession by human agency 
Succession in burned areas . 

308. Succession in lumbered areas 

309. Succession by cultivation . 
Succession by drainage 
Succession by irrig-ation 
Anomalous successions 
Perfect and imperfect successions 
Stabilization .... 

Causes and Reactions 

315. ....... 

316. Succession by preventing weathering 

317. Succession by binding aeolian soils 

318. Succession by reducing run-off and erosion 

319. Succession by filling with silt and plant remains 

320. Succession by enriching the soil . 

321. Succession by exhausting the soil 

322. Succession by the accumulation of humus . 

323. Succession by modifying atmospheric factors 
Laws of Succession 

3 2 4 

Classification and Nomenclature 

325. Basis ... . . . . . . 

326. Nomenclature 

327. Illustrations 

Investigation of Succession 

328. General rules ...... 

329. Method of alternating stages 



PAGE 

243 

244 

245 
246 
247 

247 
247 
248 
249 
249 
249 
250 
250 

251 

252 

2 53 
253 
253 
254 

254 
255 

256 

257 

258 

259 
260 

261 

262 

263 

264 

264 

267 
267 
270 

270 
271 



XVI 



CONTENTS 



330. The relict method 

THE STRUCTURE OF THE 



33I- • 

Zonation 

332. Concept 
Causes of Zonation 



333- 

334. 
335- 
336. 



Growth 
Reactions . 
Physical factors . 
Physiographic symmetry 



Kinds of Zonation 



337- 
338. 
339- 



340. 

34i. 
Alternation 

342. Concept 

343- 
344- 
345- 



Radial zonation . 
Bilateral zonation 
Vertical zonation 
Vegetation zones 



FORMATION 



Causes 
Competition 
Kinds of alternation 
The Formation in Detail 

346. The rank of the formation 

347. The parts of a formation 

348. Nomenclature of the divisions 

349. The investigation of a particular formation 
Classification and Relationship 

350. Basis ..... 
Plabitat classification . 

2. Nomenclature 
353. Developmental classification 
Regional classification 
Mixed formations 



351. 






354- 

355- 



EXPERIMENTAL VEGETATION 



356. Scope and methods 
Method of Natural Habitats 

357. Natural experiments . 
Method of Artificial Habitats 

358. Modification of habitat 



PAGE 

272 



274 

274 

275 
276 

278 

279 
280 
280 
280 
281 

283 

284 
285 
289 

292 

295 
299 

299 
300 

301 

302 

304 
304 
304 



306 

307 
307 



CONTENTS XV11 

PAGE 

359. Denuding 308 

360. Modification of the formation by transfer .... 309 
Method of Control Habitats 

361. Competition cultures . . . .. . . . .310 

362. Details of culture methods . . . . ... 311 

Glossary 314 

Bibliography -. 324 



RESEARCH METHODS IN ECOLOGY 



CHAPTER I. THE FOUNDATION OF ECOLOGY 

The Need of a System 

1. The scope of ecology. The clue to the field of ecology is found in 
the Greek word, oIkos, home. The point of view in the following treatise 
is constantly that which is inherent in the term itself. Ecology is therefore 
considered the dominant theme in the study of plants, indeed, as the central 
and vital part of botany. This statement may at first appear startling, if 
not unfounded, but mature reflection will show that all the questions of 
botanical science lead sooner or later to the two ultimate facts : plant and 
habitat. The essential truth of this has been much obscured by detached 
methods of study in physiology, morphology, and histology, which are too 
often treated as independent fields. These have suffered incomplete and 
tm symmetric development in consequence of extreme specialistic tendencies. 
Analytic methods have dominated research to the exclusion of synthetic 
ones, which, in a greatly diversified field, must be final, if botanical knowl- 
edge is something to be systematized and not merely catalogued. Physiol- 
ogy in particular has suffered at the hands of detached specialists. Orig- 
inally conceived as an inquiry into the origin and nature of plants, it has 
been developed strictly as a study of plant activities. It all but ignores the 
physical factors that control function, and the organs and tissues that 
reflect it. 

2. Ecology and physiology. There can be little question in regard to the 
essential identity of physiology and ecology. This is evident when it is 
clearly seen that the present difference between the two fields is superficial. 
Ecology has been largely the descriptive study of vegetation ; physiology 
has concerned itself with function ; but, when carefully analyzed, both are 
seen to rest upon the same foundation. In each, the development is incom- 
plete : ecology has so far been merely superficial, physiology too highly spe- 
cialized. The one. is chaotic and unsystematized, the other too often a 
minute study of function, under abnormal circumstances. The greatest need 
of the former is the introduction of method and system, of the latter, a 
broadening of scope and new objectives. The growing recognition of the 
identity of the two makes it desirable to anticipate their final merging, and 



2 THE FOUNDATION OF ECOLOGY 

to formulate a system that will combine the good in each, and at the same 
time eliminate superficial and extreme tendencies. In this connection, it 
becomes necessary to point out to ecologist and physiologist alike that, 
while they have been working on the confines of the same great field, each 
must familiarize himself with the work and methods of the other, before his 
preparation is complete. Both must broaden their horizons, and rearrange 
their views. The ecologist is sadly in need of the more intimate and exact 
methods of the physiologist : the latter must take his experiments into the 
field, and must recognize more fully that function is but the middleman be- 
tween habitat and plant. It seems probable that the final name for the whole 
field will be physiology, although the term ecology has distinct advantages 
of brevity and of meaning. In this event, however, it should be clearly 
recognized that, although the name remains the same, the field has become 
greatly broadened by new viewpoints and new methods. 

HISTORICAL DEVELOPMENT 

3. Geographical distribution. The systematic analysis of the great field 
of -ecology is essential to its proper development in the future. A glance 
at its history shows that, while a number of essential points of attack have 
been discovered, only one or two of these have been organized, and that 
there is still an almost entire lack of correlation and coordination between 
these. The earliest and simplest development of the subject was concerned 
with the distribution of plants. This was at first merely an off-shoot of 
taxonomy, and, in spite of the work of Humboldt and Schouw, has per- 
sisted in much of its primitive form to the present time, where it is repre- 
sented by innumerable lists and catalogues. Geographical distribution was 
grounded upon the species, a fact which early caused it to become stereo- 
typed as a statistical study of little value. This tendency was emphasized 
by the general practice of determining distribution for more or less arti- 
ficial areas, and of instituting comparisons between regions or continents 
too little known or too widely remote. The fixed character of the subject 
is conclusively shown by the fact that it still persists in almost the original 
form more than a half century after Grisebach pointed out that the forma- 
tion was the real unit of vegetation, and hence of distribution. 

4. The plant formation. The corner-stone of ecology was laid by Grise- 
bach in 1838 by his recognition of the plant formation as the fundamental 
feature of vegetation. Earlier writers, notably Linne (1737, I75 1 ); Biberg 
(1749), and Hedenberg (1754), had perceived this relation more or less 
clearly, but failed to reduce it to a definite guiding principle. This was a 
natural result of the dominance of descriptive botany in the 18th century, 



HISTORICAL DEVELOPMENT 3 

by virtue of which all other lines of botanical inquiry languished. This 
tendency had spent itself to a certain degree by the opening of the 19th 
century, and both plant distribution and plant physiology began to take 
form. The stimulus given the former by Humboldt (1807) turned the 
attention of botanists more critically to the study of vegetation as a field 
in itself, and the growing feeling for structure in the latter led to Grise- 
bach's concept of the formation, which he defined as follows : "I would 
term a group of plants which bears a definite physiognomic character, such 
as a meadow, a forest, etc., a phytogeographic formation. The latter may 
be characterized by a single social species, by a complex of dominant species 
belonging to one family, or, finally, it may show an aggregate of species, 
which, though of various taxonomic character, have a common peculiarity; 
thus, the alpine meadows consist almost exclusively of perennial herbs." 
The acceptance of the formation as the unit of vegetation took place slowly, 
but as a result of the work -of Kerner (1863), Grisebach (1872), Engler 
(1879), Hurt (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), 
and Warming (1889), this point of view came tc be more and more preva- 
lent. It was not, however, until the appearance of three works of great 
importance, "Warming (1895), Drude (1896), and Schimper (1898), that 
the concept of the formation became generally predominant. With the 
growing recognition of the formation during the last decade has appeared 
the inevitable tendency to stereotype the subject of ecology in this stage. 
The present need, in consequence, is to show very clearly that the idea of 
the formation is a fundamental, and not an ultimate one, and that the proper 
superstructure of ecology is yet to be reared upon this as the foundation. 

5. Plant succession. The fact that formations arise and disappear was 
perceived by Biberg as early as 1749, but it received slight attention until 
Steenstrup's study of the succession in the forests of Zealand (1844 prox.). 
In the development of formations, as well as in their recognition, nearly all 
workers have confined themselves to the investigation of particular changes. 
Eerg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), 
Hult (1881), Senft (1888), Warming (1890), and others have added much 
to our detailed knowledge of formational development. Notwithstanding 
the lapse of more than a half century, the study of plant successions is by 
no means a general practice among ecologists. This is a ready explanation 
of the fact that the vast field has so far yielded but few generalizations. 
Warming (1895) was the first to compile the few general principles of de- 
velopment clearly indicated up to this time. The first critical attempt to 
systematize the investigation of succession was made by Clements (1904), 
though this can be considered as little more than a beginning on account of 



4 THE FOUNDATION OF ECOLOGY 

the small number of successions so far studied. Future progress in this 
field will be conditioned not only by the more frequent study of develop- 
mental problems by working- ecologists, but also, and most especially, by 
the application of known principles of succession, and by the working out 
of new ones. 

6. Ecological phytogeography. Until recent years, the almost universal 
tendency was to give attention to formations from the standpoint of vegeta- 
tion alone. While the habitat was touched here and there by isolated work- 
ers, and plant functions were being studied intensively by physiologists, 
both were practically ignored by ecologists as a class. The appearance of 
Warming's Lehrbuch der oecologischen Pflanzengeographie (1896) and of 
Schimper's Pflanzengeographie auf physiologischer Grundlage (1898) rem- 
edied this condition in a measure by a general discussion of the habitat, and by 
emphasizing the importance of the ecological or physiological point of view. 
Despite their frank recognition of the unique value of the habitat, the major 
part of both books was necessarily given to what may be termed the general 
description of formations. For this reason, and for others arising out of 
an almost complete dearth of methods of investigation, ecology is still al- 
most entirely a floristic study in practice, although there is a universal recog- 
nition of the much greater value of the viewpoint which rests upon the 
relation between the formation and its habitat. 

7. Experimental ecology. Properly speaking, the experimental study of 
ecology dates from Bonnier 1 (1890, 1895), though it is well understood that 
experimental adjustment of plants to certain physical factors had been the 
subject of investigation before this time. The chief merit of Bonnkr's 
work, however, lies in the fact that it was done out of doors, under natural 
conditions, and for these reasons it should be regarded as the real begin- 
ning of this subject. Bonnier's experiments were made for the purpose of 
determining the effect of altitude. Culture plots of certain species were 
located in the Alps and the Pyrenees, and the results were compared with 
control cultures made in the lowlands about Paris. In 1894 he also made 

1 Bonnier, G. 

Les Plantes Arctiques Comparees aux Memes Esp£ces des Alps et des Pyrenees. 

Rev. Gen. Bot. 6:505. 1894. 
Cultures Experimentales dans les Alps et les Pyrenees. Rev. Gen. Bot. 2:514. 1890. 
Recherches Experimentales sur l'Adaptation des Plantes au Climat Alpin. Ann. 

Nat. Sci. 7:20:218. 1895. 

Bonnier, G., et Ch. Flahault 

Modifications des vegetaux sur l'influence des conditions physiques du milieu. 
Ann. Nat. Sci. 6:7:93. 1878. 



HISTORICAL DEVELOPMENT 5 

a comparative study of certain polydemic species common to the arctic 
islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these meth- 
ods are fundamental to field experiment, but the results are inconclusive, 
inasmuch as altitude is a complex of factors. As no careful study was 
made of the latter, it was manifestly impossible to refer changes and dif- 
ferences of structure to the definite cause. In a paper that has just ap- 
peared, E. S. Clements (1905) has applied the method of polydemic com- 
parison to nearly a hundred species of the Rocky mountains. In this work, 
the all-important advance has been made of determining accurately the de- 
cisive differences between the two or more habitats of the same species in 
terms of direct factors, water-content, humidity, and light. In his own 
investigations of Colorado mountain vegetation, the author has applied the 
method of field cultures by planting seeds of somewhat plastic species in 
habitats of measured value, and has thought to initiate a new line of re- 
search by applying experimental methods to the study of vegetation as an 
organism. In connection with this, there has also been developed a method 
of control experiment in the plant house under definitely measured differ- 
ences of water and light. 

8. Ecology of the habitat. Since the time of Humboldt, there have been 
desultory attempts to determine the physical factors of habitats with some 
degree of accuracy. The first real achievement in this line was in the 
measurement of light values by Wiesner in 1896. In 1898 the writer first 
began to study the structure of habitats by the determination of water- 
content, light, humidity, temperature, wind, etc., by means of instruments. 
These methods were used by one of his pupils, Thornber (1901), in the 
study of a particular formation, and by another, Hedgcock (1902), in a 
critical investigation of water-content. Two years later, similar methods 
of measuring physical factors were put into operation in connection with 
experimental evolution under control in -the plant house. E. S. Clements 
(1905), as already indicated, has made the use of factor instruments the 
foundation of a detailed study of polydemic species, i. e., those which grow 
in two or more habitats, and which are, indeed, the most perfect of all ex- 
periments in the production of new forms. In a volume in preparation 
upon the mountain vegetation of Colorado, the writer has brought the use 
of physical factor instruments to a logical conclusion, and has made the 
study of the habitat the basis of the whole work. Out of this investigation 
has come a new concept of vegetation (Clements 1904), namely, that it is 
to be regarded as a complex organism with structures and with functions 
susceptible of exact methods of study. 



b THE FOUNDATION OF ECOLOGY 

9. The evidence from historical development. This extremely brief 
resume of what has been accomplished in the several lines of ecological 
research makes evident the almost complete absence of correlation and of 
system. The whole field not merely lacks system, but it also demands a 
much keener perception of the relative value of the different tendencies 
already developed. It' is inevitable from the great number of tyros, and 
of dilettante students of ecology in comparison with the few specialists, that 
the surface of the field should have received all of the attention. It is, 
however, both unfortunate and unscientific that great lines of development 
should be entirely unknown to all but a few. There is no other department 
of botany in which the superficial study of more than half a century ago 
still prevails to the exclusion of better methods, many of which have been 
known for a decade or more. It is clear, then, that the imperative need of 
ecology is the proper coordination of its various points of view, and the 
working out of a definite system which will make possible a ready recogni- 
tion of that which is fundamental and of that which is merely collateral. 
The historical development, as is well understood, can furnish but a slight 
clue to this. It is a fact of common knowledge that the first development 
of any subject is general, and usually superficial also. True values come 
out clearly only after the whole field has been surveyed. For these reasons, 
as will be pointed out in detail later, the newer viewpoints are regarded as 
either the most important or the most fundamental. Experimental ecology 
will throw a flood of light upon plant structure and function, while exact 
methods of studying the habitat are practically certain of universal appli- 
cation in the future. 

PRESENT STATUS OF ECOLOGY 

10. The lack of special training. The bane of the recent development 
popularly known as ecology has been a widespread feeling that anyone can 
do ecological work, regardless of preparation. There is nothing in modern 
botany more erroneous than this feeling. The whole task of ecology is to 
find out what the living plant and the living formation are doing and have 
done in response to definite complexes of -factors, i. e., habitats. In this 
sense, ecology is practically coextensive with botany, and the student of a 
local flora who knows a few hundred species is no more competent to do 
ecological work than he is to reconstruct the phylogeny of the vegetable 
kingdom, or to explain the transmission of ancestral qualities. The com- 
prehensive and fundamental character of the subject makes a broad special 
training even more requisite than in more restricted lines of botanical in- 
quiry. The ecologist must first of all be a botanist, not a mere cataloguer 
of plants, and he must also possess a particular training in the special meth- 



PRESENT STATUS 7 

ods of ecological research. He must be familiar with the various points of 
attack in this field, and he must know the history of his subject thoroughly. 
Ecology affords the most striking example of the prevalent evil of Ameri- 
can botanical study, i. e., an indifference to, or an ignorance of the literature 
of the subject. The trouble is much aggravated here, however, by the 
breadth of the field, and the common assumption that a special training is 
unnecessary, if not, indeed, superfluous. Ignorance of the important eco- 
logical literature has been a most fertile source of crude and superficial 
studies, a condition that will become more apparent as these fields are 
worked again by carefully trained investigators. 

11. Descriptive ecology. The stage of development of the subject at the 
present time may be designated as descriptive ecology, for purposes of dis- 
cussion merely. This is concerned with the superficial description of vege- 
tation in general terms, and results from the fact that the development has 
begun on the surface, and has scarcely penetrated beneath it. The organic 
connection between ecology and floristic has produced an erroneous impres- 
sion as to the relative value of the two. Floristic has required little knowl- 
edge, and less preparation: it lends itself with insidious ease to chance jour- 
neys or to vacation trips, the fruits of which are found in vague descriptive 
articles, and in the multiplication of fictitious formations. While there is 
good reason that a record should be left of any serious reconnaissance, even 
though it be of a few weeks' duration, the resulting lists and descriptive 
articles can have only the most rudimentary value, and it is absurd to regard 
them as ecological contributions at all. No statement admits of stronger 
emphasis, and there is none that should be taken more closely to heart by 
botanists who have supposed that they were doing ecological work. An 
almost equally fertile source of valueless work is the independent treatment 
of a restricted local area. The great readiness with which floristic lists 
and descriptions can be made has led many a botanist, working in a small 
area, or passing hurriedly through an extended region, to try his hand at 
formation making. From this practice have resulted scores of so-called 
formations, which are mere patches, consocies, or stages in development, 
or which have, indeed, no existence other than in the minds of their dis- 
coverers. The misleading definiteness which a. photograph seems to give 
a bit of vegetation has been responsible for a surplus of photographic for- 
mations, which have no counterparts in nature. Indispensable as the photo- 
graph is to any systematic record of vegetation, its use up to the present 
time has but too often served to bring it into disrepute. There has been a 
marked tendency to apply the current methods of descriptive botany to 
vegetation, and to regard every slightly different piece of the floral covering 



8 THE FOUNDATION OF ECOLOGY 

as a formation. No method can yield results further from the truth. It is 
evident that the recognition and limitation of formations should be left abso- 
lutely to the broadly trained specialist, who has a thorough preparation by 
virtue of having acquainted himself carefully with the development and 
structure of typical formations over large areas. 

12. The value of floristic. In what has been said above, there is no in- 
tent to decry the value of floristic. The skilled workman can spare the 
material which he is fashioning as readily as the ecologist can work without 
an accurate knowledge of the genera and species which make up a particular 
vegetation. Some botanists whose knowledge of ecology is that of the study 
or the laboratory have maintained that it is possible to investigate vege- 
tation without knowing the plants which compose it. Ecology is to be 
wrought out in the field, however, and the field ecologist — none other, in- 
deed, should bear the name — understands that floristic alone can furnish 
the crude material which takes form under his hands. It is the absolute 
need of a thorough acquaintance with the flora of a region which makes it 
impossible for a traveler to obtain anything of real ecological value in his 
first journey through a country. As the very first step, he must gain at 
least a fair knowledge of the floristic, which will alone take the major part 
of one or more growing seasons. This information the student of a local 
flora already has at the tip of his tongue ; in itself it can not constitute a 
contribution to ecology, but merely the basis for one. In this connection, 
moreover, it can not be used independently, but becomes of value only after 
an acquaintance with a wide field. Floristic study and floristic lists, then, 
are indispensable, but to be of real value their proper function must be 
clearly recognized. They do not constitute ecology. 

13. Reconnaissance and investigation. In striving to indicate the true 
value and worth of ecological study, it becomes necessary to draw a definite 
line between what we may term reconnaissance and investigation. By the 
former is understood the preliminary survey of a region, extending over 
one or two years. The objects of such a survey are to obtain a compre- 
hensive general knowledge of the topography and vegetation of the region, 
and of its relation to the other regions about it. The chief purpose, how- 
ever, is to gain a good working acquaintance with the flora : a reconnais- 
sance to be of value must do this at all events. Certain general facts will 
inevitably appear during this process, but they will invariably need the con- 
firmation of future study. It would be an advantage to real ecology if 
reconnaissance were to confine itself entirely to the matter of making a 
careful floristic survey. Investigation begins when the inquiry is directed 



PRESENT STATUS 9 

to the habitat, or to the development and structure of the formation which 
it bears, i. e., when it takes up the manifold problems of the oiko?. Such 
a study must be based upon floristic, but the latter becomes a part of in- 
vestigation only in so far as it leads to it. Standing by itself, it is not 
ecological research : it is the preparation for it. This distinction deserves 
careful thought. The numerous recruits to ecology have turned their at- 
tention to what lay nearest to hand, with little question as to its value, or to 
where it might lead. The result has been to make reconnaissance far out- 
weigh investigation in amount, and to give it a value which properly be- 
longs to the latter. Furthermore, this mistaken conception has in many 
cases, without doubt, prevented its leading to valuable research work. 

14. Resident investigation. Obviously, if reconnaissance is a superficial 
survey, and investigation thorough extensive study, an important distinction 
between them is in the time required. While one may well be the result of 
a journey of some duration, the other is essentially dependent upon resi- 
dence. In the past the great disparity between the. size of the field and the 
number of workers has made resident study too often an ideal, but in the 
future it will be increasingly the case that a particular region w T ill be worked 
by a trained ecologist resident in it. This may never be altogether true of 
inaccessible and sterile portions of the globe. It -may be pointed out, how- 
ever, that, between the tropics and the poles, residence during the summer 
or growing period is in essence continuous residence. In the ultimate an- 
alysis, winter conditions have of course some influence upon the develop- 
ment of vegetation during the summer, but the important problems which a 
vegetation presents must be w r orked out during the period of development. 
For temperate, arctic, and alpine regions, then, repeated study during the 
growing period for a term of years has practically all the advantages of 
continuous residence. For all practical purposes, it is resident study. 

15. The dangers of a restricted field. In the resident study of a par- 
ticular region, the temptation to make an intensive investigation of a cir- 
cumscribed area is very strong. The limits imposed by distance are alone 
a sufficient explanation of this, but it is greatly increased by the inclination 
toward detailed study for which a small field offers opportunity. This 
temptation can be overcome only by a general preliminary study of the 
larger region in which the particular field is located. The broader outlook 
gained in this way will throw needed light upon many obscure facts of the 
latter, and at the same time it will act as a necessary corrective of the ten- 
dency to consider the problems of the local field in a detached manner, and 
to magnify the value of the distinctions made and the results obtained. 






IO THE FOUNDATION OF ECOLOGY 

Such a general survey has the purpose and value of a reconnaissance, and 
is always the first step in the accurate and detailed investigation of the 
local area or formation. Each corrects the extreme tendency of the other, 
and thorough comprehensive work can be done only by combining the two 
methods. When the field of inquiry is a large area or covers a whole re- 
gion, the procedure should be essentially the same. A third stage must be 
added, however, in which a more careful survey is made of the entire field 
in the light of the thorough study of the local area. The writer's methods 
in the investigation of the Colorado vegetation illustrate this procedure. 
The summers of 1896, 1897, 1898 were devoted to reconnaissance; those of 
1 899-1904 were given to detailed and comprehensive study by instrument 
and quadrat of a highly diversified, representative area less than 20 miles 
square, while the work of the final summer will be the application of the 
results obtained in this localized area to the region traversed from 1896-98. 
This is practically the application of methods of precision to an area of more 
than 100,000 square miles. It also serves to call attention to another point 
not properly appreciated as yet by those who would do ecological work. 
This is the need of taking up field problems as a result of serious fore- 
thought, and not as a matter of accident or mere propinquity. A carefully 
matured plan of attack which contemplates an expenditure of time and 
energy for a number of years will yield results of value, no matter how 
much attention an area may have received. On the other hand, an aimless 
or hurried excursion into the least known or richest of regions will lead to 
nothing -but a waste of time, especially upon the part of the ecologist, who 
must read the articles which result, if only for the purpose of making sure 
that there is nothing in them. 

APPLICATIONS OF ECOLOGY 

16. The subjects touched by ecology. The applications of ecological 
methods and results to other departments of botany, and to other fields of 
research are numerous. Many of these are both intimate and fundamental, 
and give promise of affording new and extremely fruitful points of view. 
It has already been indicated that ecology bears the closest of relations to 
morphology and histology on the one side, and to physiology on the other — 
that it is, indeed, nothing but a rational field physiology, which regards 
form and function as inseparable phenomena. The closeness with which it 
touches plant pathology follows directly from this, as pathology is nothing 
more than abnormal form and functioning. Experimental work in ecology 
is purely a study of evolution, and the facts of the latter are the materials 
with which taxonomy deals. Forestry has already been termed "applied 



APPLICATIONS 1 1 

ecology" and in its scientific aspects, which are its foundation, it is precisely 
the ecology of woody plants, and of the vegetation which they constitute. 
Apart from botany, the physical side of ecology is largely a question of soil 
physics, and of physiography. On the other hand, vegetation is coming 
more and more to be regarded as a fundamental factor in zoogeography 
and in sociology. Furthermore, with respect to the latter, it will be pointed 
out below that the principles of association which have been determined for 
plants, viz., invasion, succession, zonation, and alternation, apply with almost 
equal force to man. 

17. Physiology and pathology. The effect of ecology in emphasizing the 
intrinsically close connection between physiology and morphology has al- 
ready been mentioned. Its influence in normalizing the former by forcing 
it into the field as the place for experiment, and by directing the chief at- 
tention to the plant as an organism rather than a complex of organs, is also 
rapidly coming to be felt. Ecology will doubtless exert a corrective in- 
fluence upon pathology in the near future. This is inevitable as the latter 
ceases to be the merely formal study of specific pathogenic organisms, and 
turns its attention to the cause of all abnormality, which is to be found in 
the habitat, whether this be physical, as when the water-content is low, or 
biotic, when a parasitic fungus is present. The relative ease with which 
specific diseases can be studied has helped to obscure the essential fact that 
the approach to pathology must be through physiology. Much indeed of 
the observational physiology of the laboratories has been pathology, and it 
will be impossible to draw a clear line between them until precise experi- 
ment in the habitat has come into vogue. 

18. Experimental evolution. As a result of the extremely fragmentary 
character of the geological record, nothing is more absolute than that there 
can be no positive knowledge of the exact origin of a form or species, ex- 
cept in those rare cases of the present day, where the whole process has 
taken place under the eye of a trained observer. The origin of the plant 
forms known at present must forever lie without the domain of direct knowl- 
edge. If it were possible by a marvel of ingenuity and patience to develop 
experimentally Myosurus from Selaginella, this would not be absolutely 
conclusive proof that • Myosurus was first derived in this way. When all 
is said, however, this would be the very best of presumptive evidence. It 
must also be recognized that this is the nearest to complete proof that we 
shall ever attain, and with this in mind it becomes apparent at once that 
evidence from experiment is of paramount importance in the study of evo- 
lution (the origin of species). 



12 THE FOUNDATION OF ECOLOGY 

The phase of experimental ecology which has to do with the plant has 
well been called experimental evolution. While this is a field almost wholly 
without development at present, there can be little question that it is to be 
one of the most fertile and important in the future. Attention will be di- 
rected first to those forms which are undergoing modification at the present 
time. The cause and direction of change will be ascertained, and its amount 
and rapidity measured by biometrical methods. The next step will be to 
actually change the habitat of representative types, and to determine for 
each the general trend of adaptation, as well as the exact details. By means 
of the methods used and the results obtained in these investigations, it will 
he possible to attack the much more difficult problem of retracing the devel- 
opment of species already definitely constituted. This will be accomplished 
by the study of the derived and the supposed ancestral form, but owing to 
the g-reat preponderance of evolution over reversion, the study of the an- 
cestral form will yield much more valuable results. 

The general application of the methods of experimental ecology will mark 
a new era in the study of evolution. There has been a surplus of literary 
investigation, but altogether too little actual experiment. The great value 
of De Vries' work lies not in the importance of the results obtained, but in 
calling attention to the unique importance of experimental methods in con- 
tributing to a knowledge of evolution. The development of the latter has 
been greatly hindered by the dearth of actual facts, and by a marked ten- 
dency to compensate for this by verbiage and dogmatism. This is well illus- 
trated by the present position of the "mutation theory," which, so far as the 
evidence available is concerned, is merely a working hypothesis. An in- 
credible amount of bias and looseness of thought have characterized the 
discussion of evolution. It is earnestly to be hoped that the future will 
bring more work and less argument, and that the literary evolutionists will 
become less and less reluctant to leave the relative merits of variation and 
mutation to experiment. 

19. Taxonomy. Taxonomy is classified evolution. It is distinct from 
descriptive botany, which is merely a cataloguing of all known forms, with 
little regard to development and relationship. The consideration of the lat- 
ter is peculiarly the problem of taxonomy, but the solution must be sought 
through experimental evolution. The first task of the latter is to determine 
the course of modification in related forms, and the relationships existing 
between them. With this information, taxonomy can group forms accord- 
ing to their rank, i. e., their descent. The same method is applicable to the 
species of a genus, and, in a less degree, perhaps, to the genera which con- 
stitute a family. The use to which it may be put in indicating family re- 



APPLICATIONS 13 

lationships will depend largely upon the gap existing between the families 
concerned. While interpretation will always play a part in taxonomy, the 
general use of experiment will leave much less opportunity for the personal 
equation than is at present the case. Taxonomy, like descriptive botany, is 
based upon the species, but, while there may exist a passable kind of de- 
scriptive botany, there can be no real taxonomy as long as the sole criterion 
of a species is the difference which any observer thinks he sees between one 
plant and another. The so-called species of to-day range in value from 
mere variations to true species which are groups of great constancy and 
definiteness. The reasons for this are obvious when one recalls that "spe- 
cies" are still the product of the herbarium, not of the field, and that the 
more intensive the study, the greater the output in "species." It would 
seem that careful field study of a form for several seasons would be the 
first requisite for the making of a species, but it is a precaution which is 
entirely ignored in the vast majority of cases. The thought of subjecting 
forms presumed to be species to conclusive test by experiment has appar- 
ently not even occurred to descriptive botanists as yet. Notwithstanding, 
there can be no serious doubt that the existing practice of re-splitting hairs 
must come to an end sooner or later. The remedy will come from without 
through the application of experimental methods in the hands of the ecol- 
ogist, and the cataloguing of slight and unrelated differences will yield to 
an ordered taxonomy. 

Experimental evolution will solve a taxonomic problem as yet untouched, 
namely, the effect of recent environment upon the production of species. It 
is well understood that some species grow in nature in various habitats 
without suffering material change, while others are modified to constitute a 
new form in each habitat. It is at once clear that these forms (or ecads) 
are of more recent descent than the species, i. e., of lower rank. It must 
also be recognized that a constant group and a highly plastic one are essen- 
tially different. If constancy is made a necessary quality of a species, one 
is a species, the other is not. If both are species, then two different kinds 
must be distinguished. Among the species of our manuals are found many 
ecads, alongside of constant and inconstant species. These can be distin- 
guished only by field experiment, and their proper coordination is possible 
only after this has been done. Indeed, the whole question of the ability or 
the inability of environmental variation to produce constant species is one 
that must be referred to repeated and long-continued experiment in the field. 

A minor service of considerable value can be rendered taxonomy by 
working over the diagnosis from the ecological standpoint. Many ecological 
facts are of real diagnostic value, while others are at least of much interest, 
and serve to direct attention to the plant as a living thing. The loose use of 



14 THE FOUNDATION OF ECOLOGY 

terms denoting abundance, which prevails in lists and manuals, should be 
replaced by the exact usage which the quadrat method has made possible 
for vegetation. The designation of habitats could be made much more 
exact, and the formation, as well as the habitat-form or ecad, and the vege- 
tation-form or phyad, should be indicated in addition. The general terms 
drawn from pollination, seed-production, and dissemination might also be 
included to advantage. 

20. Forestry, if the purely commercial aspects be disregarded, is the 
ecology of a particular kind of vegetation, the forest. Therefore, in point- 
ing out the connection between them, it is only necessary to say that what- 
ever contributes to the ecology of the forest is a contribution to forestry. 
There are, however, certain lines of inquiry which are of fundamental im- 
portance. First among these, and of primary interest from the practical 
point of view, are the questions pertaining to the distribution of forests and 
their structure. Of even greater significance are the problems of forest 
development, movement, and of reforestation, which are comprised in 
succession. The gradual invasion of the plains and prairies by the forest 
belt of the east and north is in full conformity with the laws of invasion, 
and the ecological methods to be employed here serve not merely to de- 
termine the actual conditions at present, but also to forecast them with a 
great deal of accuracy. The slow but certain development of forests on new 
soils, and their more rapid re-establishment where the woody vegetation has 
been destroyed by burning or lumbering, are ordinary phenomena of suc- 
cession, for which the ecologist has already worked out the laws, and de- 
termined the methods of investigation. Having once ascertained the original 
and adjacent vegetation and the character of the habitat, the ecologist can 
indicate with accuracy not only the character of the new forest that will 
appear, but also the nature of the antecedent formations. A full knowledge 
of the character and laws of succession will prove of the greatest value to 
the forester in all studies of forestation and reforestation. Forests which 
now seem entirely unrelated will be seen to possess the most intimate de- 
velopmental connection, and the fuller insight into the life-history gained in 
this way will have a direct bearing upon methods of conservation, etc. It 
will further show that the forester must know other vegetations as well, 
since grassland and thicket formations have an intimate influence upon the 
course of the succession, as well as upon the advance of a forest frontier. 

One of the greatest aids which modern ecology can furnish forestry,, 
however, is the method of determining the physical nature of the habitat. 
So far, foresters have been obliged to content themselves with a more or 
less superficial study of the structure of forest formations, without being 



APPLICATIONS 15 

able to do more than guess at the physical causes which control both struc- 
ture and development. This handicap is especially noticeable in the case of 
forest plantings in non-forested regions, where it has been impossible to 
estimate the chances of success, or to determine the most favorable areas 
except by actual plantations. Equipped with the proper instruments for 
measuring water-content, humidity, light and temperature, the ecologist is 
able to determine the precise conditions under which reproduction is occur- 
ring, and to ascertain what non-forested areas offer the most nearly similar 
conditions. A knowledge of habitats and the means of measuring them 
enables the forester to discover the causes which control the vegetation with 
which he is already familiar, and to forecast results otherwise hidden. Fur- 
thermore, it makes it possible for him to enter a new region and to deter- 
mine its nature and capabilities at a minimum of time and energy. 

21. Physiography. Physiographic features play an important part in de- 
termining the quantity of certain direct factors of the habitat. Perhaps a 
more important connection between physiography and ecology is to be found 
in succession. The beginning of all primary, and of many secondary suc- 
cessions is to be sought in the physiographic processes which produce new 
habitats, -or modify old ones. On the other hand, most of the reactions which 
continue successions exert a direct influence upon the form of the land. 
The most pronounced influence of terrestrial successions is found in the 
stabilization which their ultimate stages exert upon land forms, even where 
these are highly immature. The chief effect of aquatic successions is to be 
found in the "silting up" and the formation of new land, which result from 
the action of vegetation upon silt-bearing waters. The closeness of the re- 
lation between succession and the forms of the land has led to the application 
of the term "physiographic ecology" to that part of the subject which deals 
with the development of vegetation, i. e., succession. 

22. 5oil physics. This subject is as much a part of ecology as is forestry. 
It is intrinsically that subdivision of ecology which deals with the edaphic 
factors of the habitat, and their relation to the plant. Since the basis is 
physics, there has been a general tendency to overvalue the determination 
of soil properties, and to ignore the fact that these are decisive only when 
considered with reference to the living plant. As the soil contains the 
water which is the factor of greatest importance to plants, soil physics is an 
especially important part of ecology. Its methods are discussed under the 
habitat. 

23. Zoogeography. Since animals are free for the most part, and hence 
not confined so strictly to one spot as plants, their dependence upon the 



1 6 THE FOUNDATION OF ECOLOGY 

habitat is not so evident. The relation is further obscured by the fact that 
no physical factor has the direct effect upon them which water or light 
exerts upon the plant. Vegetation, indeed, as the source of food and pro- 
tection, plays a more obvious, if not a more important part. This is 
especially true of anthophilous insects, but it also holds for all herbivorous 
animals, and, through them, for carnivorous ones. The animal ecology of 
a particular region can only be properly investigated after the habitats and 
plant formations have been carefully studied. Here, as in floristics, a great 
deal can be done in the way of listing the fauna, or studying the life habits 
of its species, without any knowledge of plant ecology ; but an adequate 
study must be based upon a knowledge of the vegetation. Although animal 
formations are often poorly defined, there can be no doubt of their exist- 
ence. Frequently they coincide with plant formations, and then have very 
definite limits. They exhibit both development and structure, and are sub- 
ject to the laws of invasion, succession, zonation, and alternation, though 
these are not altogether similar to those known for plants, a fact readily 
explained by the motility of animals. Considered from the above point 
of view, zoogeography is a virgin field, and it promises great things to the 
student who approaches it with the proper training. 

24. Sociology. In its fundamental aspects, sociology is the ecology of a 
particular species of animal, and has in consequence, a similar close con- 
nection with plant ecology. The widespread migration of man and his 
social nature have resulted in the production of groups or communities 
which have much more in common with plant formations than do formations 
of other animals. The laws of association apply with especial force to 
the family, tribe, community, etc., while the laws of succession are essen- 
tially the same for both plants and man. At first thought it might seem 
that man's ability to change his dwelling-place and to modify his environ- 
ment exempts him in large measure from the influence of the habitat. The 
exemption, however, is only apparent, as the control exerted by climate, 
soil, and physiography is all but absolute, particularly when man's depend- 
ence upon vegetation, both natural and cultural, is called to mind. 

The Essentials of a System 

25. Cause and effect: habitat and plant. In seeking to lay the foundation 
for a broad and thorough system of -ecological research, it is necessary to 
scan the whole field, and to discriminate carefully between what is funda- 
mental and what is merely collateral. The chief task is to discover, if 
possible, such a guiding principle as will furnish a basis for a permanent 
and logical superstructure. In ecology, the one relation which is precedent 



ESSENTIALS OF A SYSTEM 1 7 

to all others is the one that exists between the habitat and the plant. This 
relation has long been known, but its full value has yet to be appreciated. 
It is precisely the relation that exists between cause and effect, and its fun- 
damental importance lies in the fact that all questions concerning the plant 
lead back to it ultimately. Other relations are important, but no other is 
paramount, or able to serve as the basis of ecology. Ecology sums up this 
relation of cause and effect in a single word, and it may be that this ad- 
vantage will finally cause its general acceptance as the proper name for 
this great field. 

In the further analysis of the connection between the habitat and the plant, 
it is evident that the causes or factors of the habitat act directly upon the 
plant as an individual, and at the same time upon plants as groups of in- 
dividuals. The latter in no wise decreases the importance of the plant as 
the primary effect of the habitat, but it gives form to research by making it 
possible to consider two great natural groups of phenomena, each character- 
ized by very different categories of effects. Ecology thus falls naturally 
into three great fundamental fields of inquiry : habitat, plant, and formation 
(or vegetation). To be sure, the last can be approached only through the 
plant, but as the latter is not an individual, but the unit of a complex from 
the formational standpoint, the formation itself may be regarded as a sort 
of multiple organism, which is in many ways at least a direct effect of the 
habitat. In emphasizing this fundamental relation of habitat and vegeta- 
tion, it is imperative not to ignore the fact that neither plant nor formation 
is altogether the effect of its present habitat. A third element must always 
be considered, namely, the historical fact, by which is meant the ancestral 
structure. Upon analysis, however, this is in its turn found to be the product 
of antecedent habitats, and in consequence the essential connection between 
the habitat and the plant is seen to be absolute. 

26. The place of function. In the foregoing it is understood that the 
immediate effect of the physical factors of the habitat is to be found in the 
functions of the plant, and that these determine the plant structure. Func- 
tion has so long been the especial theme of plant physiology that methods 
of investigation are numerous and well known, and it is unnecessary here 
to consider it further than to indicate its general bearing. The essential 
sequence in ecological research, then, is the one already indicated, viz., 
habitat, plant, and formation, and this will constitute the order of treatment 
in the following pages. That portion of floristic which is not mere de- 
scriptive botany belongs to the consideration of the formation, and in con- 
sequence there will be no special treatment of floristic as a subdivision of 
ecology. 



CHAPTER IT. THE HABITAT 

Concept and Analysis 

27. Definition of the habitat. The habitat is the sum of all the forces or 
factors present in a given area. It is the exact equivalent of the term en- 
vironment, though the latter is commonly used in a more general sense. As 
an ecological concept, the habitat refers to an area much more definite in 
character, and more sharply limited in extent than the habitat of species as 
indicated in the manuals. Since the careful study of habitats has scarcely 
begun, it is impossible to recognize and delimit them in an absolute sense. 
Visible topographic boundaries often exist, but in many cases, the limit, 
though actual, is not readily perceived. Contiguous habitats may be sharply 
limited, or they may pass into each other so gradually that no real line of 
demarcation can be drawn. Whatever variations they may show, however, 
all habitats agree in the possession of certain essential factors, which are 
universally present. On the other hand, a few factors are merely incidental 
and may be present or absent. The relative value and amount of these is 
probably similar for no two habitats, though the latter readily fall into 
groups with reference to the amount of some particular factor. 

28. Factors. The factors of a habitat are water-content, humidity, light, 
temperature, soil, wind, precipitation, pressure, altitude, exposure, slope, 
surface (cover), and animals. To these should be added gravity and polarity, 
which are practically uniform for all habitats, and may, in consequence, be 
ignored in this treatise. Length of season, while it plays an important part 
in vegetation, is clearly a complex and is to be treated under its constituents. 
Of the factors given, all are regularly found in each habitat, though some 
are not constantly present. The first five, water-content, humidity, light, 
temperature, and soil are the most important, and any one may well serve 
as a basis for grouping habitats into particular classes with reference to 
quantity. As will be pointed out later, however, water-content and light 
furnish the most striking differences between habitats, and offer the best 
means of classification. As habitats are inseparable from the formations 
which they bear, the discussion of the kinds of habitats is reserved for 
chapter IV. 



FACTORS 19 

Classification of Factors 

29. The nature of factors. The factors of a habitat are arranged in two 
groups according to their nature: (1) physical, (2) biotic. In the strict 
sense, the physical factors constitute the habitat proper, and are the real 
causative forces. No habitat escapes the influence of biotic factors, how- 
ever, as the formation always reacts upon it, and the influence of animals 
is usually felt in some measure. Physical factors are further grouped into 
(1) climatic and (2) edaphic, with respect to source, or, better, the medium 
in which they are found. Climatic, or atmospheric factors are humidity, 
light, temperature, wind, pressure, and precipitation. Axiomatically, the 
stimuli which they produce are especially related to the leaf. Edaphic or 
soil factors are confined to the -soil, as the term denotes, and are im- 
mediately concerned with the functions of the root. Water-content is by 
far the most important of these; the others are soil composition (nutrient- 
content), soil temperature, altitude, slope, exposure, and surface. The last 
four are of a more general character than the others, and are usually re- 
ferred to as physiographic factors. Cover, when dead, might well be placed 
among these also, but as it is little different from the living cover in effect, it 
seems most logical to refer it to biotic factors. 

30. The influence of factors. While the above classification is both ob- 
vious and convenient, a more logical and intimate grouping may be made 
upon the influence which the factor exerts. On this basis, factors are 
divided into (1) direct, (2) indirect, and (3) remote. Direct factors are 
those which act directly upon an important function of the plant and produce 
a formative effect: for example, an increase in humidity produces an im- 
mediate decrease in transpiration. They are water-content, humidity, and 
light. Other factors have a direct action: thus temperature has an im- 
mediate influence upon respiration and probably assimilation also, but it is 
not structurally formative. Wind has a direct mechanical effect upon woody 
plants, but it does not fall within our definition. Indirect factors are those 
that affect a formative function of the plant through another factor; thus 
a change in temperature causes a change in humidity and this in turn calls 
forth a change in transpiration; or, a change in soil texture increases the 
water-content, and this affects the imbibition of the root-hairs. Indirect 
factors, then, are temperature, wind, pressure, precipitation, and soil compo- 
sition. Remote factors are, for the most part, physiographic and biotic: 
they require at least two other factors to act as middlemen. Altitude affects 
plants through pressure, which modifies humidity, and hence transpiration. 
Slope determines in large degree the run-off during a rain-storm, thus 






20 THE HABITAT 

affecting water-content and the amount of water absorbed. Earthworms 
and plant parts change the texture of the soil, and thereby the water-content. 
Indirect factors often exert a remote influence -also, as may be seen in the 
effect which temperature and wind have in increasing evaporation from the 
soil, and thus reducing the water-content. This distinction between factors 
may seem insufficiently grounded. In this event, it should be noted that it 
centers the effects of all factors upon the three direct ones, water-content, 
humidity, and light. If it further be recalled that these are the only factors 
which produce qualitative structural changes, and that the classification of 
ecads and formations is based upon them, the validity of the distinction is 
clear. 

The Determination of Factors 

31. The need of exact measurement. Any serious endeavor to find in the 
habitat those causes which are producing modification in the plant and in 
vegetation can not stop with the factors merely. The next step is to de- 
termine the quantity of each. It is not sufficient to hazard a guess at this, 
or to make a rough estimate of it. Habitats differ in all degrees, and it is 
impossible to institute comparisons between them without an exact measure 
of each factor. Similarly, one can not trace the adaptations of species to 
their proper causes unless the quantity of each factor is known. It is of 
little value to know the general effect of a factor, unless it is known to 
what degree this effect is exerted. For this purpose it becomes necessary 
to appeal to instruments, in order to determine the exact amount of each 
factor that is present in a particular habitat, and hence to determine the 
ratio between the stimulus and the amount of structural adjustment which 
results. The employment of instruments of precision is clearly indispen- 
sable for the task which we have set for ecology, and every student that 
intends to strike at the root of the subject, and to make lasting contributions 
to it, must familiarize himself with instrumental methods. One great benefit 
will accrue to ecology as soon as this fact is generally recognized. The use 
of instruments and the application of results obtained from them demand 
much patience and seriousness of purpose upon the part of the student. As 
a consequence, there will be a general exodus from ecology of those that 
have been attracted to it as the latest botanical fad, and have done so much 
to bring it into disrepute. 

32. The value of meteorological methods. At the outset there must be 
a very clear understanding that weather records and readings have only a 
very general value. This is in spite of the fact that the instruments em- 
ployed are of standard precision. An important reason for this lack of 



FACTORS 21 

value is that readings are not made in a particular habitat ; as a rule, indeed, 
they are made in towns and cities, and hence are far removed from masses 
of vegetation. They are usually taken at considerable heights, and give but 
a general indication of the conditions at the level of vegetation. The chief 
difficulty, however, is that the factors observed at weather stations — tem- 
perature, pressure, wind, and precipitation — are those which have the least 
value for the ecologist. It is true that a knowledge of the temperature and 
rainfall of at great region will afford some idea of the general character of 
its vegetation. A proper understanding of such a vegetation is, however, 
to be gained only through the exact study of its component formations. 
Ecology has already incurred sufficient censure as a subject composed of 
very general ideas, and the use of meteorological data, which can never be 
connected definitely with anything in the plant or the formation, should be 
discontinued. This must not be understood to mean that meteorological in- 
struments can not be used in the proper place and manner, i. e., in the habitat. 

33. Habitat determination. It is self-evident that determinations of 
factors by instruments can only be of value in the habitat where they are 
made. In other words, a habitat is a unit for purposes of measuring its 
factors, and measures of one habitat have no exact value in another. This 
fact can not be overstated. Thus, -while it is perfectly legitimate, and indeed 
highly desirable, to locate thermographs in different mountain zones for 
ascertaining the rate at which temperature decreases with altitude, the 
data obtained in this way are not directly applicable in explanation of plant 
or formation changes, except when the same species occurs at different al- 
titudes. Special methods are valuable and often absolutely necessary, but 
in view of the fact that the plant as well as the formation is the definite 
product of a definite habitat, the fundamental rule in instrumentation is 
that complete readings must be made within a habitat for that habitat alone. 
This necessarily presupposes a certain preliminary acquaintance with the 
habitat to be investigated, as it is imperative that the station for making 
readings be located well within the formation, in order to avoid transition 
conditions. In vegetation, there are as many habitats as formations, and 
in addition a large number of new and denuded habitats upon which suc- 
cessions have not yet started; a knowledge of each formation or succession 
must rest ultimately upon the factors of its particular habitat. 

34. Determinable and efficient differences. The instruments employed in 
studying habitats can not be too exact, as there is no> adequate knowledge as 
yet concerning the real differences which exist between related or con- 
tiguous formations. This is particularly true of differences which are 



22 THE HABITAT 

efficient in producing a recognizable structural change in plant or formation. 
Investigations made by the writer have shown that standard instruments 
will measure differences of quantity quite too small to produce a visible re- 
action. Efficient differences are not the same for different factors, and 
perhaps also for the same factor when found in various combinations. They 
vary widely for different species, being in direct relation to the plasticity of 
the latter. The point necessary to bear in mind in formulating methods for 
habitat investigation and in making use of instruments is thai! standard in- 
struments should be used for the very reason that we do not yet know the 
relation between determinable and efficient differences. On the other hand, 
it is unnecessary to insist upon absolute exactness as soon as it is found that 
the determinable difference lies well within the efficient one. This by no 
means indicates that instruments are not to be carefully standardized and 
frequently checked, or that accurate readings should not be made. It 
means that a slight margin of error may be permitted in a machine which 
registers well within the efficient difference for that factor, and that instru- 
ments that read to the last degree of nicety are not absolutely necessary. In 
the fundamental work of determining efficient differences, however, instru- 
ments can not have too great precision. Moreover, these differences must be 
based upon the most plastic species of a formation, and the readings must 
be made under normal conditions. 

Instrumentation 

35. Methods. In the field use of instruments two methods have been de- 
veloped. The first in point of time was the method of simple instruments, 
devised especially for class work, and capable of being used only where a 
number of trained students are available. The method of automatic instru- 
ments was an immediate outgrowth of this, due to the necessity which con- 
fronts the solitary investigator of being in different habitats at the same 
time. In the gradual evolution of this subject, it has become possible to 
combine the two methods in such a way as to retain all the advantages of 
the automatic method, and most of those of the method of simple 
instruments. 

36, Method of simple instruments. By simple instruments are denoted 
those that do not record, but must be read by the observer at the time. 
They are standard instruments of precision, but possess the disadvantage 
of requiring an observer for each one. They are well illustrated by the 
thermometers and psychrometers used by the Weather Bureau. In the 
hands of trained -observers the results obtained are unimpeachable; in fact, 
standard simple instruments must be constantly employed to check automatic 



INSTRUMENTS 23 

ones. As physical factors vary greatly through the day and through the 
year, it is all-important that the readings in habitats which are being com- 
pared should be made at the same instant. This requires a number of ob- 
servers ; as many as twelve stations have been read at one time, and there is 
of course no limit to the number. It is very important, also, that observers be 
carefully trained in the handling of instruments, and in reading them ac- 
curately and intelligently at the proper moment. In practice it has been 
found impossible to do such work in elementary classes, and, even in using 
small advanced classes, prolonged drill has been necessary before trust- 
worthy results could be obtained. When a class has once been thoroughly 
trained in making accurate simultaneous readings, there is practically no 
limit, other than that set by time, to the valuable work that can be done, 
both in instruction and investigation. 

37. Method of automatic instruments. The solitary investigator must 
replace trained helpers by automatic instruments or ecographs. These have 
the very great advantages of giving continuous simultaneous records for 
long periods, and of having no personal equation. They must be regulated 
and checked, to be sure, but as this is all done by the same person, the error 
is negligible. There is nothing more satisfactory in resident investigation 
than a series of accurate recording instruments in various habitats. Eco- 
graphs have two disadvantages. The chief perhaps is cost. The expense 
of a single "battery" which will record light, water-content, humidity, and 
temperature is about $250. Another difficulty is that they can be used 
only within a few miles of the base, since they require attention every week 
for regulation, change of record, etc. While this means that ecographs in 
their present form are not adapted to reconnaissance, this is not a real dis- 
advantage, as the scattered observations possible on such a journey can best 
be made by simple instruments. 

38. Combined methods. The best results by far are to be obtained by the 
combined use of simple and automatic instruments. This is particularly true 
in research, but it applies also to class instruction. The ecographs afford a 
continuous, accurate basal record, to which a single reading made at any 
time or place can be readily referred for comparison. On the other hand, 
it is an easy matter to carry a full complement of simple instruments on the 
daily field trips, and to make accurate readings in a score or more of forma- 
tions in a single day. An isolated reading, especially of a climatic factor, 
has little or no value in itself, but when it can be compared with a reading 
made at the same time in the base station by an ecograph, it is the equivalent 
of an automatic reading. This method renders a set of simple instruments 



24 THE HABITAT 

more desirable for a long trip or reconnaissance than a battery of automatic 
ones. It is practically impossible to carry the latter into the field, and in any 
event a continuous record is out of the question. As there are other tasks 
at such times also, it becomes evident that the taking of single readings 
which can be compared with a continuous record offers the most satisfactory 
solution. 

Construction and Use of Instruments 

39. The selection of instruments. In selecting and devising instruments 
for the investigation of physical factors, emphasis has first been laid upon 
accuracy. This is the result of a feeling that it is better to have instruments 
that read too minutely than those which do not make distinctions that are 
sufficiently close, particularly until more has been learned about efficient 
differences. On the other hand, no hesitation has been felt in employing in- 
struments which are not absolutely accurate, when it was clear that the 
error was less than the efficient difference. Similarly, the margin of error 
practically eliminates itself in the case of simultaneous comparative readings, 
when the instruments have been checked to the same standard. Simplicity 
of construction and operation are of great importance, especially in saving 
time where a large number of instruments are in operation. Expense is 
likewise to be carefully considered. It is impossible to have too many in- 
struments, but cost practically determines the number that can be obtained. 
It is further necessary to secure or invent both simple and automatic instru- 
ments for all factors, except such invariable ones as altitude, slope, etc. 
Simple instruments must be of a kind that can be easily carried, and so con- 
structed that they can be used at a minimum of risk. The sling psychro- 
meter, for example, is very readily broken in field use, and it has been 
replaced by a protected modification, the rotating form. 

In describing the construction and operation of the many factor instru- 
ments, there has been no attempt to make the treatment exhaustive. Those 
instruments which the author has found of greatest value in his own work 
are given precedence, and the manner of using them is described in detail. 
Other instruments of value are also considered, though with greater brevity. 
Some of the most complex and expensive ones have been ignored, as it is 
altogether improbable that they can come into general use in their present 
form. While the conviction is felt that the methods described below will 
enable the most advanced investigators to carry on thorough work, it is 
hoped that they will be seen to be so fundamental, and so attractive, that they 
will appeal to all who are planning serious ecological study. 



INSTRUMENTS 



25 



WATER-CONTENT 

40. Value of different instruments. The paramount importance of water- 
content as a direct factor in the modification of plant form and distribution 
gives a fundamental value to the methods used for its determination. Au- 
tomatic instruments for ascertaining the water in the soil are costly, in ad- 
dition to being complicated, and often inaccurate. For these reasons, much 
attention has been given to developing the simpler but more reliable methods 
in which a soil-borer or geotome is used. The latter is simple, inexpensive, 
and accurate. It can be carried easily upon daily trips or upon longer re- 
connaissances, and is 

always ready for instant 
use. In the determination 
of physiological water-con- 
tent, it is •practically indis- 
pensable. Indeed, the 
readiness with which geo- 
tome determinations of 
water-content can be made 
should hasten the universal 
recognition of the fact that 
it is the available, and not 
the total amount of water 
in the soil, which deter- 
mines the effect upon the 
plant. 

Geotome Methods 

41. The geotome. In its 

simplest form, the geotome 
is merely a stout iron tube 
with a sharp cutting edge 
at one end and a firmly 
attached handle at the 




Fig. 1. Geotomes and soil can. 



other. The length is variable and is primarily determined by the location 
of the active root surface of the plant. In xerophytic habitats, generally a 
longer tube is necessary than in mesophytic ones. The bore is largely 
determined by the character of the soil ; for example, a larger one is neces- 
sary for gravel than for loam. Tubes of small bore also tend to pack the 
soil below them, and to give a correspondingly incomplete core. The best 
results have been obtained with geotomes of ^2-1 inch tube. Each geotome 



26 



THE HABITAT 




has a removable rod, flattened into a disk at one end, and bent at the other, 
for forcing-out the core after it has been cut from the soil. Sets of geotomes 
have been made in lengths of 5, 10, 12, 15, 20, and 25 inches. The 12- and 
15-inch forms have been commonly used for herbaceous formations and 
layers. They are marked in inches so that a sample of any lesser depth may 
be readily taken. Such a device is very necessary for gravel soils and in 
mountain regions, where the subsoil of rock lies close to the surface. 

42. Soil borers. There is a large variety of soil borers to choose from, 
but none have been found as simple and satisfactory for relatively shallow 
readings as the geotome just described. For deep-rooted plants, many 
xerophytes, shrubs, and trees, borers of the -auger type are necessary. 
These are large and heavy, and of necessity slow in operation. They can 
not well be carried in an ordinary outfit of instruments, and the size of the 

soil sample itself precludes the use of such 
instruments far from the base station, except 
on trips made expressly for obtaining samples 
from deep-seated layers. For depths from 
two to eight feet, the Fraenkel borer is per- 
haps the most satisfactory, except for the 
coarser gravels ; it costs $14 -or $20 according 
to the length. For greater depths, or when 
a larger core is desirable, the Bausch & Lomb 
borer, number 16536, which costs $5.25, 
should be made use of. This is a ponderous 
affair and can be employed only on special 
occasions. On account of the size of samples 
obtained by these borers, it is usually most 
satisfactory to take a small sample from the 
core at different depths. Frequently, indeed, 
a hand trowel may be readily used to obtain 
a good sample at a particular depth. 

43. Taking samples of soil. In -obtaining soil samples, the usual practice 
is to remove the air-dried surface, noting its depth, and to sink the geotome 
with a slow, gentle, boring movement, in order to avoid packing the soil. 
This difficulty is further obviated by deep notches with sharp, beveled edges 
which are cut at the lower end. In obtaining a fifteen-inch core, there is 
also less compression if it be cut five inches at a time. Repeated tests have 
shown, however, that the single compressed sample is practically as trust- 
worthy as the one made in sections. The water-content of the former 
constantly fell within .5 per cent of that of the latter, and both varied less 



Fig. 2. Fraenkel 
soil borer. 



Fig. 3. Ameri- 
can soil borer. 



WATER-CONTENT 



27 



than 1 per cent from the dug sample used as a check. As soon as dug, the 
core is pressed out of the geotome by the plunger directly into an air-tight 
soil can. Bottles may be used as containers, but tin cans are lighter and 
more durable. Aluminum cans have been devised foi^this purpose, but on 
account of the expense, "Antikamnia" cans have been used instead. These 
are tested, and those that are not water-tight are rejected, although it has 
been found that, even in these, ordinary soils do not lose an appreciable 
amount of water in twenty-four hours. The lid should be screwed on as 
quickly as possible, and, as an added precaution, the cans are kept in a close 
case until they have been weighed. The cans are numbered consecutively 
on both lid and side in such a way that the number may be read at a glance. 
The numbers are painted, as a label wears off too rapidly, and scratched 
numbers are not quickly 
discerned. 

44. Weighing. Al- 
though soil samples 
have -been kept in tight 
cans outside of cases 
for several days without 
losing a milligram of 
moisture, the safest plan 
is to make it a rule to 
weigh cans as quickly 
as possible after bring- 
ing them in from the 
field. Moreover, when 
delicate balances are 
available, it is a good 
practice to weigh to the 
milligram. At remote 
bases, however, and 
particularly in the field, 
and on reconnaissance, where delicate, expensive instruments are out of 
place, coarser balances, which weigh accurately to one centigram, give 
satisfactory results. The study of efficient water-content values has already 
gone far enough to indicate that differences less than 1 per cent are neg- 
ligible. Indeed, the soil variation in a single square meter is often as 
great as this. The greatest difference possible. in the third place, i. e., that 
of 9 milligrams, does not produce a difference of .1 of 1 per cent in 
the water-content value. In consequence, such strong portable balances 
as Bausch & Lomb 12308 ($2), which can be carried anywhere, give entirely 




Fig. 4. Field balance. 



28 THE HABITAT 

reliable results. The best procedure is to weigh the soil with the can. 
Turning the soil out upon the pan or upon paper obviates one weighing, 
but there is always some slight loss, and the chances of serious mishap are 
many. After weighing, the sample is dried as rapidly as possible in a 
water bath or oven. At a temperature of ioo° C. this is accomplished 
ordinarily in twenty-four hours ; the most tenacious clays require a longer 
time, or a higher temperature. High temperatures should be avoided, 
however, for soils that contain much leaf mould or other organic matter, 
in order that this may not be destroyed. When it is necessary on trips, 
soil samples can be dried in the sun or even in the air. This usually takes 
several days, however, -and a test weighing is generally required before one 
can be certain that the moisture is entirely gone. The weighing of the 
•dried soil is made as before, and the can is carefully brushed out and 
weighed. The weight of aluminum cans may be determined once for all, 
but with painted cans it has been the practice to weigh them each time. 

45. Computation. The most direct method of expressing the water- 
content is by per cents figured upon the moist soil as a basis. The ideal 
way would be to determine the actual amount of water per unit volume, 
but as this would necessitate weighing one unit volume at least in every 
habitat studied, as a preliminary step, it is not practicable. The actual 
process of computation is extremely simple. The weight of the dried 
sample, zc/ 1 , is subtracted from the weight of the original sample, w, and 
the weight of the can, w 2 , is likewise subtracted from w. The first result 
is then divided by the second, giving the per cent of water, or the physical 

w — w^ 
water-content. The formula is : 5 = W. The result is expressed pref- 
er — 7tr 

erably in grams per hundred grams of moist soil; thus 20/100, from which 
the per cent of water-content may readily be figured on the basis of dry 
or moist soil. 

46. Time and location of readings. Owing to the daily change in the 
amount of soil water due. to evaporation, gravity, and rainfall, an isolated 
determination of water-content has very little value. It is a primary re- 
quisite that a basis for comparison be established by making (1) a series 
of readings in the same place, (2) a series at practically the same time in 
a number of different places or habitats, or (3) by combining the two 
methods, and following the daily changes of a series of stations through- 
out an entire season, or at least for a period sufficient to determine the 
approximate maximum and minimum. The last procedure can hardly be 
carried out except at a base station, but here it is practically indispensable. 
It has been followed both at Lincoln and at Minnehaha, resulting in a 
basal series for each place that -is of the greatest importance. When such a 



WATER-CONTENT 29 

base already exists, or, better, while it is being established, scattered readings 
may be used somewhat profitably. As a practical working rule, however, 
it is most convenient and satisfactory to make all determinations consecu- 
tively, i. e., in a series of stations or of successive days. Under ordinary 
conditions, the time of day at which a particular sample is taken is of lit- 
tle importance, as the variation during a day is usually slight. This does 
not hold for exposed wet soils, and especially for soils which have just been 
wetted by rains. In all comparative series, however, the samples should be 
taken at the same hour whenever possible. This is particularly necessary 
when it is desired to ascertain the daily decrease of water-content in the 
same spot. In the case of a series of stations, these should be read always 
in the same order, at the same time of day, and as rapidly as possible. When 
a daily station series is being run, i. e., a series by days and stations both, 
the daily reading for each place should fall at the same time. While there 
are certain advantages in making readings either early or late in the day, 
they may be made at any time if the above precautions are followed. 

47. Location of readings. Samples should invariably be taken in spots 
which are both typical and normal, especially when they are to be used as 
representative of a particular area or habitat. A slight change in slope, soil- 
composition, in the amount of dead or living cover, etc., will produce con- 
siderable change in the amount of water present. Where habitat and 
formation are uniform, fewer precautions are necessary. This is a rare cir- 
cumstance, and as a rule determinations must be made wherever appreciable 
differences are in evidence. The problem is simpler when readings are taken 
with reference to the structure or modifications of a particular species, but 
even here, check-readings in several places are of great value. The varia- 
tion of water in 'a spot apparently uniform has been found to be slight in the 
prairies and the mountains. In taking three samples in spots a few inches to 
several feet apart, the difference in the amount of water has rarely ex- 
ceeded 1 per cent, which is practically negligible. Gardner 1 found that 16 
samples taken to a depth of 3 inches, in as many different portions of a 
carefully prepared, denuded soil plot, showed a variation of 7^ per cent. 
This is partially explained by the shallowness of the samples, but even then 
the results of the two investigations are in serious conflict and indicate that 
the question needs especial study. It should be further pointed out that all 
readings should be made well within a particular area, and not near its 
edge, and that, in the case of large diversified habitats, it is the consocies 
and the society which indicate the obvious variations in the structure of 
the habitat. 

- 1 Gardner, F. D. The Electrical. Methods of Moisture Determination in Soils. 
Bull. Div. Soils, 12:12. 1893. 



3Q THE HABITAT 

48. Depth of samples. The general rule is that the depth of soil sam- 
ples is determined by the layer to which the roots penetrate. The prac- 
tice is to remove the air-dried -surface in which no roots are found, and to 
take a sample to the proper depth. This method is open to some objection, 
as the actively absorbing root surfaces are often localized. There is no 
practical way of taking account of this as yet, except in the case of deep- 
rooted xerophytes and woody plants. It is practicable to determine the 
location -of the active root area of a particular plant and hence the water- 
content of the soil layer, but in most formations, roots penetrate to such 
different depths that a sample which includes the greater part of the distance 
concerned is satisfactory. Some knowledge of the soil of a formation is also 
necessary, since shallow soils do not require as deep samples as others. The 
same is true of shaded soils without reference to their depth, and, in large 
measure, of soils supplied with telluric water. In all cases, it is highly de- 
sirable to have numerous control-samples at different depths. The normal 
cores are 12 or 15 inches; control-samples are taken every 5 inches to the 
depth desired, and in some cases 3-inch sections are made. It has been 
found a great saving of time to combine these methods. A 5-inch sample is 
taken and placed in one can, then a second one, and a third in like manner. 
In this way the water-content of each 5-inch layer -is determined, and from 
the combined weight the total content is readily ascertained. 

49. Check and control instruments. A number of instruments throw 
much light upon the general relations of soil water. The rain-gauge, or 
ombrometer, measures the periodical replenishment of the water supply, 
and has a direct bearing upon seasonal variation. The atmonieter affords 
a clue to the daily decrease of water by evaporation, and thus supplements 
the rain-gauge. The run-off gauge enables one to establish a direct connec- 
tion between water-content and the slope and character of the surface. The 
amount and rapidity of absorption are determined by means of a simple in- 
strument termed a rhoptometer. The gravitation water of a soil is ascer- 
tained by a hizometer, and some clue to the hygroscopic and capillary water 
may be obtained by an artificial osmotic cell. All of these are of importance 
because they serve to explain the water-content of a particular soil with 
especial reference to the other factors of the habitat. It is evident that none 
of them can actually be used in exact determinations of the amount of 
water, and they will be considered under the factors with which they are 
more immediately concerned. 

Physical and Physiological Water 

50. The availability of soil water. The amount of water present in a 
soil is no real index to the influence of water-content as a factor of the 



WATER-CONTENT 31 

habitat. All soils contain more water than can be absorbed by the plants 
which grow in them. This residual water, which is not available for use, 
varies for different soils. It is greatest in the compact soils, such as clay 
and loam, and least in the loose ones, as sand- and gravel. It differs, but to a 
much less degree, from one species to another. A plant of xerophytic tend- 
ency is naturally able to remove more water from the same soil than one of 
mesophytic or hydrophytic character. As the species of a particular forma- 
tion owe their association chiefly to their common relation to the water- 
content of the habitat, this difference is of little importance in the field. In 
comparing the structure of formations, and especially that of the plants which 
are found in them, the need to distinguish the available water from the total 
amount is imperative. Thus, water-contents of 15 per cent in gravel and in 
clay are in no wise comparable. A coarse gravel containing 15 per cent 
of water is practically saturated. The plants which grow upon it are 
mesophytes of a strong hydrophytic tendency, and they are able to use 14^2 
or all but .5 out of the 15 per cent of water. In a compact clay, only 3^ 
of the 15 per cent are available, and the plants growing in it are marked 
xerophytes. It is evident that a knowledge merely of the physical water- 
content is actually misleading in such cases, and this holds true of com- 
parisons of any soils which differ considerably in texture. After one has 
determined the physiological water for the great groups of soils, it is more 
or less possible to estimate the amounts in the various types of each. ( As 
an analysis is necessary to show how close soils are in texture, this is 
little better than a guess, and for accurate work it is indispensable that the 
available water be determined for each habitat. Within the same formation, 
however, after this has once been carefully ascertained, it is perfectly satis- 
factory to convert physical water-content into available by subtracting the 
non-available water, which under normal conditions in the field remains 
practically the same. 

The importance of knowing the available water is even greater in those 
habitats in which salts, acids, cold, or other factors than the molecular at- 
traction of soil-particles increase the amount of water which the plant can 
not absorb. Few careful investigations of such soils have yet been made, 
and the relation of available to non-available water in them is almost en- 
tirely unknown. It is probable that the phenomena in some of these will be 
found to be produced by other factors. 

51. Terms. The terms, physiological water-content, and physical water- 
content, are awkward and not altogether clear in their application. It is 
here proposed to replace them by short words which will refer directly to 
the availability of the soil water for absorption by the plant. Accordingly, 



32 THE HABITAT 

the total amount of water in the soil is divided into the available and the 
non-available water-content. The terms suggested for these are respect- 
ively, holard (6'Aos, whole, apoov, water), chresard (x/o^sis, use), and echard (€'x (0 > 
to withhold). 

52. Chresard determinations under control. The determination of the 
chresard in the field is attended with peculiar difficulties. In consequence, 
the method of obtaining it under control will first be described. The 
inquiry may be made with reference to soils in general or to the soil of 
a particular formation. In the last case, if the plants used are from the same 
formation, the results will have almost the value of a field determination. 
When no definite habitat is the subject of investigation, an actual soil, and 
not an artificial mixture, should be used, and the plants employed should be 
mesophytes. The individual plants are grown from seeds in the proper 
soils, and are repotted sufficiently often to keep the roots away from the 
surface. The last transfer is made to a pot large enough to permit the plant 
to become full-grown without crowding the roots. The pot should be glazed 
inside and out in order to prevent the escape of moisture. This interferes 
slightly with the aeration of the soil, but it will not cause any real difficulty. 
The plant is watered in such a way as to make the growth as normal as 
possible. After it has become well established, three soil samples are taken 
in such a manner that they will give the variation in different parts of the 
pot. One is taken near the plant, the second midway between the plant and 
the edge of the pot, and the third near the edge. The depth is determined 
by the size of the pot and the position of the roots. The holard is determined 
for these in the usual way, but the result is expressed with reference to ioo 
grams of dry soil ; the average is taken as representative. The soil is then 
allowed to dry out slowly, as sudden drouth will sometimes impair the power 
of absorption and a plant will wilt although considerable available water 
remains. Plants often wilt in the field daily for several successive hot dry 
days, and become completely turgid again during the night. If the drying 
out takes place slowly, the plant will not recover after it has once begun 
to wilt. The proper time to make the second reading is indicated by the 
pronounced wilting of the leaves and shoots. Complete wilting occurs, as 
a rule, only after the younger parts have drawn for some time upon the 
watery tissues of the stem and root, by which time evaporation has con- 
siderably deceased the water in the soil. It is a well-known fact that young 
leaves do not wilt easily, especially in watery or succulent plants. Three 
samples are again taken and the average water-content determined as 
above. This is the non-available water or the echard. The latter is then 
computed on the basis of ioo grams of dry soil, and this result is subtracted 



WATER-CONTENT 33 

from the holard to give the chresard in grams for each ioo grams of dry 
weight. The chresard may also be expressed with respect to ioo grams of 
moist soil. As a final precaution in basal work, it is advisable to determine 
the chresard for six individuals of the same species under as nearly the 
same conditions as possible. When it is desired, however, to find the 
average chresard for a particular soil, it is necessary to employ various 
species representing diverse phyads and ecads. Such an investigation is 
necessarily very complicated, and must be made the subject of special 
inquiry. 

53. Chresard readings in the field. The especial difficulties which must 
be overcome in the field are the exclusion of rain and dew and the cutting 
Off of the capillary water. It is evident, of course, that experiments of this 
sort must also be entirely free from outside disturbance. The choice of an 
area depends upon the scope of the study. If the chresard is sought for a 
particular consocies, the block of soil to be studied should show several 
species which are fairly representative. In case the chresard of a certain 
species is to be obtained, this species alone need be present, but it should 
be represented by several individuals. Check plots are desirable in either 
event, and at least two or three which are as nearly uniform as possible 
should be chosen. The size and depth of the soil block depends upon the 
plants concerned. It must be large enough that the roots of the particular 
individuals under investigation are not disturbed. There is a limit to the 
size of the mass that can be handled readily, and in consequence the test 
plants must not be too large or too deeply rooted. The task of cutting out 
the soil block requires a spade with a long sharp blade. After ascertaining 
the spread and depth of the roots, the block is cut so that a margin of several 
inches free from the roots concerned is left on the sides and bottom. If 
the block is to be lifted out of place, so that the sides are exposed to evapor- 
ation, this allowance should be greater. In some cases, it may be found 
more convenient to dig the plant up, place it in a large pot, and put the latter 
back in the hole. As a general practice, however, this is much less satis- 
factory. 

After the block has been cut, it may be moved if the soil is sufficiently com- 
pact, and then allowed to dry out in its own formation or elsewhere. The 
results are most valuable in the first case, though it is often an advantage 
to remove blocks cut from shade or wet formations to dry, sunny stations 
where they will dry more rapidly. The most satisfactory and natural 
method, however, is to leave the block in place, and to prevent the reestab- 
lishment of capillary action by enclosing' it within plates. This is accom- 
plished by slipping thin sheet-iron plates into position along the cut surfaces. 



34 



THE HABITAT 



The plate for the bottom should be somewhat wider than the block, and is 
slipped into place by raising the block if the soil is not too loose; in the 
latter event, it is carefully driven in. The side plates are then pushed down 
to meet the former. The size of the plates depends upon the block; in 
general, plates of I, 2, and 3 feet square, with the bottom plates a trifle 
larger, are the most serviceable. Access of rain and dew is prevented by an 
awning of heavy canvas which projects far enough beyond each side of the 
block to prevent wetting. The height will depend of course upon the size of 
the plants. The awning must be used only when rain or heavy dew is threat- 
ened, as the shade which it produces changes the power of the plant to draw 
water from the soil. 

The time necessary to cause wilting varies with the habitat and the 
weather. When the block is large and in position, two or three weeks are 
required. This period of drying incidentally furnishes an excellent oppor- 
tunity for determining the rate at which the particular soil loses water. 
The holard sample is taken daily for 'several days before the block is cut 
out, in order to obtain an average, care being taken of course to avoid a 
period of extreme weather. The echard samples are taken as soon as the 
wilting is sufficient to indicate that the limit of available water is reached. 
The air-dry soil above the roots is first removed. The treatment of the 
samples and the computation of the chresard are as previously indicated. 

54. Chresard values of different soils. The following table gives the 
water-content values of six representative soils. The per cents of holard 
(at saturation) and of echard are those determined by Hedgcock 1 with six 
mesophytes as test plants for each soil. The chresard has been computed 
directly from these. 





HOLARD 


ECHARD 


CHRESARD 


Sand 


14.3 
47.4 
59.3 
64.1 
65.3 
68.5 


12.6 
32.5 
37.1 
39.1 
39.6 
40.8 


.3 
9.3 
10.1 
10.9 
11.9 
16.2 


.25 
6.3 
6.4 
6.6 
7.2 
9.6 


14 

38.1 

49.2 

53.2 

53.4 

52.3 


12.3 


Clay 

Loess 

Loam 


26.2 
30.7 
32.5 


Humus 


32.4 


Saline 


31.2 



The first column indicates the per cent based upon the dry weight, the second upon 
the weight of the moist soil. 

While these can not be considered absolute for a particular soil other than 
the ones investigated, they are found to correspond somewhat closely to 
the results obtained for other soils of the respective groups. For accurate 

1 Hedgcock, G. G. The Relation of the Water-Content of the Soil to Certain 
Plants, Principally Mesophytes. Rep. Bot. Surv. Nebr., 6:48. 1902. 



WATER-CONTENT 



35 



research, the chresard must of course be ascertained for each formation with 
respect to its peculiar plants and soil. The influence of the ecad in more or 
less determining the echard is also shown by Hedgcock, who found that 
floating plants wilt at 25 per cent, amphibious ones at 15-20 per cent, 
mesophytes at 6-12 per cent, and mesophytic xerophytes at 3-6 per cent. 
The echard is also somewhat higher for shade plants than for heliophytes. 

Records and Results 

55. The field record. It is superfluous to point out that a definite form 
for field records saves much time and prevents many mistakes. The exact 
form may be left to personal taste, but there are certain features which are 
essential. Many of these are evident, while others may seem unnecessary; 
all, however, have been proved by experience to have some value in saving 
time or in preventing confusion. The two fundamental maxims of field work 
are that nothing is too trivial to be of importance, and that no detail should 
be entrusted to the memory. The field record should contain in unmis- 
takable terms all that the field has yielded. These statements apply with 
especial force to water-content, in many senses the most important of 
physical factors. The precise character of the record depends upon the way 
in which the readings are made, whether scattered or in series. As the 
day-station series is of the greatest importance, the record is adapted for it 
especially, but it will also serve for all readings. The record is chronolog- 
ical, since this is the only convenient method for the field. A proper form 
for a field record of water-content is the following: 





4J 

P 


0% 


a 
.2 


>> 

s 
H 


'0 
to 


V 

S 

CO 


HOLARD 


ECHARD 


T3 

u 
03 
CO 

V 

u 

-C 

U 

16 
11 

7 


NOTES 


cd 


Weighings 


* 


Weighings 


$ 

10 
5 

1 


>> 

M 

c/3 




1st 


2d 


Can 


1st 


2d 


Can 


cS.ctf 


10 
17 
40 


2/8/04 
1* 

(1 


Spruce forest 
Spruce forest 
Gravel slide. 


Jack Brook 
Milky Way 
Hiawatha 


Mertensiare 

Gentianare 

Asterare 


Loam 

Arnold 
% gravel 

Gravel 


10 

10:2 

2:10 


58.7 

64.25 

78.55 


50.1 
57.5 
74.3 


25.52 
21.35 

22 85 


2 

16 
8 


58.7 
64.25 

78.55 


53.41 
59.6 

74.85 


25.52 
21.35 

22.85 


Cloudy 
Cloudy 
Cloudy 








A general designation of the soil-composition is a material aid, especiallv 
where there is a difference in the core. For example, in a mountain forest 
or meadow, the upper layer will usually be mold, the lower sand or gravel. 
A careful estimate of the relation between the two throws much light upon 
the chresard. Under "sample" the number taken to reach the desired depth, 
if more than one, is indicated by placing the number before the depth, thus 
2:10. When two or more full cores are included in the same sample for a 



36 THE HABITAT 

check, the order is 10:2. It has already been shown, however, that these 
precautions are not necessary for ordinary purposes. In computing the 
holard and echard, there is no need to show the figuring, if the process is 
checked and then proved. Notes upon sky conditions aid in explaining the 
daily decrease in water-content. The amount of rain and the period during 
which it falls are of great importance in understanding the fluctuations of 
the holard. Under community it is highly desirable to have a list of all the 
species, but it is impossible to include this in the table, and a glance at the 
formation list will show them. The form indicated above serves for a day- 
station series, a dailv series in one station for anv number of check series 
in one spot, and for scattered readings. In many cases the echard will not 
be determined, but on account of its primary importance, there should be a 
space for it, especially since it may be desirable to determine it at some 
later time. 

56. The permanent record. This should be kept by formation, or if the 
latter exhibits well-defined associations, the formational record may be 
divided accordingly. This may seem an unnecessary expenditure of time, 
but a slight experience in finding the water-content values of a particular 
habitat, when scattered through a chronological field record, will be con- 
vincing. The form of permanent record is the same as for the field, except 
that the column for the formation and that for the society are often un- 
necessary. 

57. Sums and means. From the great difficulty of determining the abso- 
lute water-content, and of obtaining a standard of comparison between soils 
on account of the varying ratio between bulk and weight, water-content sums 
are impracticable. For the same reasons, means of actual water-content 
are practically impossible, and the mean water-content must be expressed in 
per cents. Daily readings are not essential to a satisfactory mean. In fact, 
a single reading at each extreme enables one to approximate the real mean 
very closely ; thus, the average of 26 readings in the prairie formation is 
18 per cent. The extremes are 5 per cent and 28 per cent, and their 
average 16.5 per cent. A few readings properly scattered through moist 
and dry periods will give a reliable mean, as will also* a series of daily read- 
ings from one heavy rain through a long dry period. The one difficulty with 
the last method is that such periods can not well be determined beforehand. 
Means permit ready comparison between habitats, but in connecting the 
modifications of a species with water-content as a cause, the extremes are 
significant as indicating the range of conditions. Furthermore, the ex- 
tremes, i. e., 5 per cent and 28 per cent, make it possible to approximate 



WATER-CONTENT 37 

the mean, 18 per cent, while the latter gives little or no clue to the extremes. 
It is hardly necessary to state that means and extremes should be deter- 
mined for a certain habitat, or particular area of it, and that the results may 
he expressed with reference to holard and chresard. 

58. Curves. The value of graphic methods and the details of plotting 
curves are reserved for a particular section. It will suffice in this place to 
indicate the water-content curves that are of especial value. Simple curves 
are made with regard to time, place, or depth. The day curve shows the 
fluctuations of the water-content of one station from day to day or from 
time to time. The station curve indicates the variation in water from sta- 
tion to station, while the depth curve represents the different values at var- 
ious depths in the same station. These may be combined on the same sheet 
in such a way that the station curves of each day may be compared directly. 
Similar combinations may be used for comparing the day curves, or the 
depth curves of different stations, but these are of less importance. A 
combination of curves which is of the greatest value is one which admits of 
direct comparison between the station curves of saturation, holard, 
chresard, and echard. 

HUMIDITY 

59. Instruments. As a direct factor, humidity is intimately connected 
with water-content in determining the structure and distribution of plants. 
The one is in control of water loss ; the other regulates water supply. 
Humidity as a climatic factor undergoes greater fluctuation in the same 
habitat, and the efficient difference is correspondingly greater. Accordingly, 
simple instruments are less valuable than automatic ones, since a continuous 
record is essential to a proper understanding of the real influence of 
humidity. As is the rule, however, the use of simple instruments, when they 
can be referred to an ecographic basis, greatly extends the field which can 
be studied. In investigation, both psychrometer and psychrograph have 
their proper place. In the consideration of simple instruments for obtaining 
humidity values, an arbitrary distinction is made between psychrometers 
and hygrometers. The former consist of a wet and a dry bulb thermometer, 
while the latter make use of a hygroscopic awn, hair, or other object. \ 

Psychrometers 

60. Kinds. There are three kinds of psychrometer, the sling, the cog, 
and the stationary. All consist of a wet bulb and a dry bulb thermometer 
set in a case ; the first two are designed to be moved or whirled in the air. 
The same principle is applied in each, viz., that evaporation produces a 



38 



THE HABITAT 




£5 



f§ 



3 



r 



w 

Fig. 5. Sling psy- 
chrometer. 



decrease in temperature proportional to the amount of 
moisture in the air. The dry bulb thermometer is an 
ordinary thermometer, while the wet bulb is covered with 
a cloth that can be moistened. The former indicates the 
normal temperature of the air, the latter gives the re- 
duced temperature due to evaporation. The relative 
humidity of the air is ascertained by means of the proper 
tables, from two terms, i. e., the air temperature and 
the amount of reduction shown by the wet bulb. The 
sling and the cog psychrometers alone are in general 
use. The stationary form has been found to be unre- 
liable, because the moisture, as it evaporates from the 
wet bulb, is not removed, and, in consequence, hinders 
evaporation to the proper degree. 

61. The sling psych rometer. The standard form of 
this is shown in the illustration, and is the one used by 
the Weather Bureau. This instrument can be obtained 
from H. J. Green, 1191 Bedford Ave., Brooklyn, or 
Julien P. Friez, 107 E. German St., Baltimore, at a cost 
of $5. It consists of a metal frame to which are firmly 
attached two accurately standardized thermometers, 
reading usually from -30 ° to 130 . The frame is attached 
at the uppermost end to a handle in such fashion that 
it swings freely. The wet bulb thermometer is placed 
lower, chiefly to aid in wetting the cloth more readily. 
The cloth for the wet bulb should be always of the same 
texture and quality; the standard used by the Weather 
Bureau can be obtained from the instrument makers. A 
slight difference in texture makes no appreciable error, 
but the results obtained with different instruments and 
by different observers will be more trustworthy and 
comparable if the same cloth be used in all cases. The 
jacket for the wet bulb may be sewed in the form of a 
close-fitting bag, which soon shrinks and clings tightly. 
It may be made in the field by wrapping the cloth 
so that the edges just overlap, and tying it tightly above 
and below the bulb. In either case, a single layer of 
cloth alone must be used. The cloth becomes soiled or 
thin after a few months' constant use and should be 
replaced. It is a wise precaution to carry a small piece 
of psychrometer cloth in the field outfit. 



HUMIDITY 



39 



62. Readings. All observations should be made facing the wind, and the 
observer should move one or two steps during the reading to prevent the 
possibility of error. The cloth of the wet bulb is moistened with water by 
means of a brush, or, much better, it is dipped directly into a bottle of water. 
Distilled water is preferable, as it contains no dissolved material to accumu- 
late in the cloth. Tap-water and the water of streams may be used with- 
out appreciable error, if the cloth is changed somewhat more frequently. 
The temperature of the water 
is practically negligible under 
ordinary conditions. Read- 
ings can be made more 
quickly, however, when the 
temperature is not too far 
from that of the air. The 
psychrometer is held 
firmly and swung rapidly 
through the air when the 
space is • not too confined. 
Where there is danger of 
breakage, it is swung back 
and forth through a short arc, 
pendulum-fashion. As the 
reading must be made when 
the mercury of the wet bulb 
reaches the lowest point, the 
instrument is stopped from 
time to time and the position 
of the column noted. The 
lowest point is often indicated 
by the tendency of the mer- 
cury to remain stationary; 
as a rule it can be noted with certainty when the next glance shows a rise 
in the column. In following the movement, and especially in noting the 
final reading, great care must be taken to make the latter before the mer- 
cury begins to rise. For this reason it is desirable to shade the psychro- 
meter with the body when looking at it, and to take pains not to breathe 
upon the bulbs nor to bring them too near the body. At the moment when 
the wet bulb registers the lowest point, the dry bulb should be read and the 
results recorded. 

63. Cog psychrometer. This instrument, commonly called the "egg- 
beater" psychrometer, has been devised to obviate certain disadvantages of 




Fig. 6. Cog psychrometer. 



40 THE HABITAT 

the sling psychrometer in field work, and has entirely supplanted the latter 
in the writer's own studies. It is smaller, more compact, and the danger of 
breaking in carriage or in use is almost nil. It has the great advantage of 
making it possible to take readings in a layer of air less than two inches 
in thickness, and in any position. Fairly accurate results can even be ob- 
tained from transpiring leaves. The instrument can readily be made by 
a good mechanic, at a cost for materials of $1.75, which is less than half the 
price for the sling form. A single drawback exists in the use of short, 
Centigrade thermometers, inasmuch as tables of relative humidity are 
usually expressed in Fahrenheit. It is a simple matter, however, to con- 
vert Centigrade degrees into Fahrenheit, mentally, or the difficulty may be 
avoided by the conversion table shown on page 47, or by constructing a 
Centigrade series of humidity tables. The fact that the wet and dry bulbs 
revolve in the same path has raised a doubt concerning the accuracy of the 
results obtained with this instrument. Repeated comparisons with the 
sling psychrometer have not only removed this doubt completely, but have 
also proved that the standardization of the thermometers has been efficient. 

64. Construction and use. A convenient form of egg-beater is the Lyon 
(Albany, New York), in which the revolving plates can be readily removed, 
leaving the axis and the frame. The thermometers used are of the short 
Centigrade type. They are /\.y 2 inches long and read from -5 to 50 °. 
Eimer and Amend, 205 Third Ave., New York city, furnish them at 75 
cents each. The thermometers are carefully standardized and compared, 
and then grouped in pairs that read together. Each pair is used to con- 
struct a particular psychrometer. Each thermometer is strongly wired to 
one side of the frame, pieces of felt being used to protect the tube and in- 
crease the contact. The frame is also bent at the base angles to permit 
free circulation of air about the thermometer bulbs. The bulb of one ther- 
mometer is covered with the proper cloth, and the psychrometer is finished. 
Since the frame revolves with the thermometers, it is necessary to pour the 
water on the wet bulb, or to employ a pipette or brush. The thermometer 
bulbs are placed in the layer to be studied, and the frame rotated at an even 
rate and with moderate rapidity. The observation is further made as in the 
case of the sling psychrometer. As the circle of rotation is less than three 
inches in diameter, and the layer less than an inch, in place of nearly three 
feet for the sling form, the instrument should not be moved at all for ex- 
tremely localized readings, but it must be moved considerably, a foot or 
more, if it is desirable to obtain a more general reading. 

65. Hygrometers. While there are instruments designed to indicate the 
humidity by means of a hygroscopic substance, not one of them seems to 



HUMIDITY 



41 



be of sufficient accuracy for use in ecological study. The difficulty is that 
the hygroscopic reaction is inconstant, rather than that the instruments are 
not sufficiently sensitive. A number of hygrometers have been tested, and 
in all the error has been found to be great, varying usually from 10—20 per 
cent. In the middle of the scale they sometimes read more accurately, but 
toward either extreme they are very inexact. It seems probable that an 
accurate hygrometer can be constructed only after the model of the Draper 
psychrograph. Its weight and bulk would make it an impossible instrument 
for field trips, and the ex- 
pense of one would provide 
a dozen psychrometers. 
In consequence, it does not 
seem too sweeping to say 
that no hygrometer can 
furnish trustworthy results. 
Of simple instruments for 
humidity, the psychro- 
meter alone can be trusted 
to give reliable readings. 
Crova's hygrometer, used 
by Hesselmarm, is not a 
hygrometer in the sense in- 
dicated. As it is much less 
convenient to handle and 
to operate than the cog 
psychrometer, it is not 
necessary to describe it. 

Psychrographs 

66. The Draper psy= 
chrograph. A year's trial 
of the Draper psychro- 
graph in field and plant- 
house has left little ques- 
tion of its accuracy and its great usefulness. Essentially, it consists of a 
band of fine catgut strings, which are sensitive to changes in the moisture- 
content of the air. The variations in the length of the band are com- 
municated to a long pointer carrying an inking pen. The latter traces the 
record in per cent of relative humidity on a graduated paper disk, which is 
practically the face of an eight-day clock. The whole is enclosed in a metal 
case with a glass front. A glance at the illustration will show the general 




Fig. 7. Draper psychrograph. 



4 2 



THE HABITAT 



structure of the instrument. Continued psychrometric tests demonstrate 
that the margin of error is well within the efficient difference for humidity, 
which is taken to be 5 per cent. In the field tests of the past summer, two 
psychrographs placed side by side in the same habitat did not vary 1 per 
cent from each other. The same instruments when in different habitats 
did not deviate more than 1 per cent from the psychrometric values, ex- 
cept when the air approached saturation. For humidities above 90 per cent, 
the deviation is considerable, but as these are temporary and incident upon 
rainfall, the error is not serious. For humidities varying from 10-85 P er 
cent, the psychrograph is practically as accurate as the psychrometer. Per 
cents below 10 are rare, and no tests have been made for them. 




Fig. 8. Instrument shelter, showing thermograph and psychrograph 

in position. 



67. Placing the instrument. The psychrograph should be located in a 
place where the circulation of the air is typical of the station observed. A 
satisfactory shelter will screen the instrument from sun and rain, and at 
the same time permit the air to pass freely through the perforations of 
the metal case. The form shown in figure 8 meets both of these condi- 
tions. A desirable modification is effected by fastening a strip about the 
cover of such depth as to prevent the sun's rays from striking the case ex- 
cept when the sun is near the horizon. A cross block is fastened on the 



HUMIDITY 43 

post of the shelter after being exactly leveled. The psychrograph rests upon 
this block, which is three feet above the ground in order to avoid the in- 
fluence of radiation. The instrument is held in position by slipping the 
eye over a small-headed nail driven obliquely. It does not hang from the 
latter, but must rest firmly upon the cross block. The post is set to a depth 
that prevents oscillation in the wind, which is liable to obscure the record. 
In shallow mountain soils stability is attained by fastening a broad board 
at the base of the post before setting it. When two or more psychrographs 
are established in different habitats, great pains are taken to set them up 
in exactly the same way. The shelters are alike, the height above the soil 
the same, and the instruments all face the south. 

68. Regulating and operating the instrument. When two or more psy- 
chrographs are to be used in series, they must be compared with each other 
in the same spot for several days until they run exactly together with re- 
spect to per cent of humidity and to time. During this comparison they are 
checked by the psychrometer and so regulated that they register the proper 
humidity. When a single instrument is used alone as the basis to which 
simple readings may be referred, all regulating may well be done after the 
instrument is in position. This is a simple process; it is accomplished by 
obtaining the relative humidity beneath the shelter and at the proper 
height by a psychrometer. The pen hand is then moved to the proper line 
on the disk by means of the screws at its base. These are reached by re- 
moving the lettered glass face. The thumbscrew on the side opposite the 
direction in which the pen is to move is released, and the opposite screw 
simultaneously tightened, until the pen remains upon the proper line. 
Experience has proved that the record sheet should be correctly labeled and 
dated before being placed on the disk. In the press of field duties, records 
labeled after removal are liable to be confused. It is likewise a great saving 
of time to write the date of the month in the margin of each segment. Care 
is taken to place the sheet on the disk in the same position each time; this 
can easily be done by seeing that the sharp point on the disk penetrates the 
same spot on the paper. A single drop of ink in the pen will usually give 
the most satisfactory line. A thin line is read most accurately. If the pen 
point is too fine, however, the ink does not flow readily, and the point 
should be slightly blunted by means of a file. More often the line is too 
broad and the pen must be carefully pointed. Occasionally the pen does not 
touch the sheet, and it becomes necessary to bend the hand slightly. This 
is a frequent difficulty if the records are folded or wrinkled, and conse- 
quently the sheets should always be kept flat. 



44 THE HABITAT 

69. The weekly visit. Psychrographs must be visited, checked, rewound, 
and inked every week. Whenever possible this should be done regularly 
at a specified day and hour. This is especially desirable if the same record 
sheet is used for more than one week. Time and energy are saved by a fixed 
order for the various tasks to be done at each visit. After opening the 
instrument the disk is removed, and the clock wound, and, if need be, regu- 
lated. The record sheet is replaced, the disk again put on the clock arbor, 
and the pen replenished with a drop of ink. A psychrometer reading is 
made, and the results in terms of relative humidity noted at the proper 
place on the disk sheet. If the psychrograph vary more than I per cent, it 
is adjusted to read accurately. In practice it has been found a great con- 
venience to keep each record sheet in position for three weeks, and the 
time may easily be extended to four. In this event, the pen is carefully 
cleane'd with blotting paper at each visit, and is then refilled with an ink 
of different color. To prevent confusion, the three different colored inks 
are always used in the same order, red for the first week, blue for the second, 
and. green for the third. The advantages of this plan are obvious : fewer 
records are used and less time is spent in changing them. The records of 
several weeks are side by side instead of on separate sheets, and in working 
over the season's results, it is necessary to handle but a third as many 
sheets. 

The Draper psychrograph is made by the Draper Manufacturing Com- 
pany, 152 Front St., New York city. The price is $30. A few record 
sheets and a bottle of red ink are furnished with it. Additional records can 
be obtained at 3 cents each. The inks are 25-50 cents per bottle, depending 
upon the color. 

Humidity Readings and Records 

70. The time of readings. If simple instruments alone are used for 
determining humidity, readings are practically without value unless made 
simultaneously through several stations, or successively at one. When it is 
possible to combine these, and to make psychrometer readings at different 
habitats for each hour of the day, or at the same hour for several days, the 
series is of very great value. Single readings are unreliable on account of 
the hourly and daily variations of humidity, but when these changes are 
recorded by a psychrograph, such readings at once become of use, whether 
made in the same habitat with the recording instrument or elsewhere. In 
the latter case, one reading will tell little about the normal humidity of the 
habitat, but several make a close estimate possible. When a series of 
psychrographs is in use, accurate observations can be made to advantage 
anywhere at any time. As a rule, however, it has been found most con- 



HUMIDITY 



45 



venient to make simple readings at 6:00 a.m., i :oo p.m., and 6:00 p.m., as 
these hours afford much evidence in regard to the daily range. A good time 
also is that at which the temperature maximum occurs each day, but this is 
movable and in the press of field work can rarely be taken advantage of. A 
very fair idea of the daily mean humidity is obtainable by averaging the 
readings made at the hours already indicated. The comparison of single 
readings with the psychrograph record should not be made at a time when 
a rapid change is occurring, as the automatic instrument does not respond 
immediately. Such a condition is usually represented by a sudden rain, and 
is naturally not a satisfactory time for single readings in any event. 

71. Place and height. As stated above, the psy- 
chrograph is placed three feet above the surface of 
the ground in making readings for the comparison 
of stations. In low, herbaceous formations, the in- 
strument is usually placed within a few inches of the 
soil in order to record the humidity of the air in 
which the plants are growing. In forest formations, 
the moisture often varies considerably in the differ- 
ent layers. This variation is easify determined by 
simultaneous psychrometer readings in the several 
layers, or, if occasion warrants, a series of psychro- 
graphs may be used. In field work the rule has been 
to make observations with the psychrometer at 6 
feet, 3 feet, and the surface of the soil, but the read- 
ing at the height of 3 feet is ordinarily sufficient. 
Humidity varies so easily that several readings in 
different parts of one formation are often desirable. 
In comparing different formations, the readings 
should be made in corresponding situations, for ex- 
ample, in the densest portion of each. 




Fig. 9. Atmometer. 



72. Check instruments. Humidity is so readily 
affected by temperature, wind, and pressure, that 
a knowledge of these factors is essential to an un- 
derstanding of its fluctuations. Pressure, disregarding daily variation, 
is taken account of in the tables for ascertaining relative humidity, 
and is determined once for all when the altitude of a station has 
been carefully established. The temperature is obtained directly from the 
dry bulb reading. Its value is fundamental, as the amount of moisture in 
a given space is directly affected by it ; like pressure, it also is taken account 
of in the formula. The movement of the air has an immediate influence 



46 THE HABITAT 

upon moisture by mixing the air of different habitats and layers. So far 
as the plant is concerned, it has practically the effect of increasing or de- 
creasing the humidity by the removal of the air above it. Thus, while the 
anemometer can furnish no direct evidence as to the amount of variation, 
it is of aid in explaining the reason for it. Likewise, the rate of evaporation 
as indicated by a series of atmometers, affords a ready method of estimat- 
ing the comparative effect of humidity in different habitats. Potometers 
and other instruments for measuring transpiration throw much light upon 
humidity values. Since they are concerned with the response of the plant 
to humidity, they are considered in the following chapter. 

73. Humidity tables. To ascertain the relative humidity, the difference 
between the wet and dry bulb readings is obtained. This, with the dry bulb 
temperature, is referred to the tables, where the corresponding humidity is 
found. A variation in temperature has less effect than a variation in the 
difference ; in consequence, the dry bulb reading is expressed in the nearest 
unit, and the difference reckoned to the nearest .5. The humidity varies with 
the air pressure. Hence, the altitude must be determined for the base station, 
and for all others that show much change in elevation. Within the ordinary 
range of growing-period temperatures, the effect of pressure is not great. 
For all ordinary cases, it suffices to compute tables for pressures of 30, 29, 
27, 25, and 23 inches. The following table indicates the decrease in pres- 
sure which is due to altitude. 



ALTITUDE 


PRESSURE 




Feet 


Meters 


Inches 


Centimeters 








30 




76 


910 


277 


29 




73.5 


1850 


574 


28 




71 


2820 


860 


27 




68.5 


3820 


1165 


26 




66 


4850 


1477 


25 




63.5 


5910 


1792 


24 




61 


7010 


2138 


23 




58.5 


8150 


2485 


22 




56 


9330 


2845 


21 




53.5 


10550 


3217 


20 




51 


13170 


4016 


18 




46 


16000 


4880 


16 




41 



The fluctuations of pressure due to weather are usually so slight that 
their influence may be disregarded. An excellent series of tables of relative 
humidity is found in Marvin's Psychrometric Tables, published by the U. S. 
Weather Bureau, and to be obtained from the Division of Publications, 
Washington, D. C, for 10 cents. A convenient field form is made by remov- 
ing the portion containing the tables of relative humidity, and binding it in 
stiff oilcloth. 



HUMIDITY 



47 



74. Sums, means, and 
curves. An approximate hu- 
midity sum can be obtained 
by adding the absolute hu- 
midities for each of the 
twenty-four hours, and ex- 
pressing- the results in grains 
per cubic foot. It is possible 
to establish a general ratio 
between this sum and the 
transpiration sum of the 
plant, but its value is not 
great at present. Means of 
absolute and of relative hu- 
midity are readily determine 
able from the psychrograph 
records; the latter are the 
most useful. The mean of 
relative humidity for the 
twenty-four hours of a day 
is the average of the twenty- 
four hour humidities. 
From these means the sea- 
sonal mean is computed in 
the same manner. A close 
approximation, usually with- 
in i degree, may be obtained 
in either case by averaging 
the maximum and minimum 
for the period concerned. 
Various kinds of curves are 
of value in representing 
variation in humidity. Ob- 
viously, these must be 
derived from the psychro- 
graph, or from the psychro- 
meter when the series is 
sufficiently complete. The 
level curve indicates the 
variation in different sta- 
tions at the same time. 



Boiling 212 



C 

*0J5 
OJ 



Melting Ice 32 



210 

205 

200 

105 

190 

185 

180 

175 

170 

165 

160 

155 

150 

145 

140 

135 

130 

J 25 

120 

115 

110 

105 

100 

95 

90 

85 

80 

75 

70 

65 

60 

55 

50 

45 

40 

35 



30 
25 
20 
15 
10 
5 


5 

10 

15 

20 



-100 

95 

•90 

35 

■80 

75. 
■70 

65 

60. 

55 
•50 

45 

40 

35 

ZQ 

25 

20 

15 

10, 

5 



s 

io' 
IS* 

20 
25 



-3 

GO 

<D 

O) 

n 



Fig. 10. Conversion scale for temperatures. 



4 8 



THE HABITAT 



These may be combined in a series for the comparison of readings made at 
various heights in the stations. The day or point curve shows the fluctuations 
during the day of one point, and the station curve the variation at different 
heights in the same station. The curves of successive days or of different 
stations may of course be combined on the same sheet for comparison. Level 
and station curves based upon mean relative humidities are especially 
valuable. 

75. Records. A field form is obviously unnecessary for the psychrograph. 
The record sheets constitute both a field and permanent record. The alti- 
tude and other constant features of the station and the list of species, etc., 
are entered on the back of the first record sheet, or, better, they are noted 
in the permanent formation record. For psychrometer readings, whether 
single or in series, the following record form is employed : 





U 

3 
O 

W 


ccl 


3 
.2 

+J 

10 


V 

3 
< 


3 

° 3 


o be 
1) <u 


jQ 
>> 

u 

p 


3 


'3 


a 

3 
.3 

<L1 
Pi 


a 

3 

w 
tn 
e3 


a' 

3 

05 

.O 


NOTES 


OS 

Q 


>> 

M 


3 
'3 




15/8/'04 


6:20 a.m. 


Spruce 


Brook bank 


2500 m 


Mertensiare 


1ft. 


51° 


46° 


5 


72$ 


63$ 


2.9 


Clear 








u 


" 


Half gravel 


Hiawatha . . 


" 


Asterare . . . 


" 


56° 


49° 


7 


64$ 


63$ 


3.0 


" 








(< 


6:45 p.m. 


Spruce 


Brook bank 


(4 


Mertensiare 


" 


54° 


52° 


2 


89$ 


69$ 


4.2 


«■ 


2cc. 





" 


" 


Half gravel 


Hiawatha . . 


C< 


Asterare . . . 


(( 


56° 


52° 


4 


79$ 


69$ 


4.0 


" 


2cc. 






On page 47 is given a table for the conversion of Centigrade into Fahren- 
heit temperatures. This may be done mentally by means of the formula 

^=4x9+32°. 

LIGHT 



76. Methods. All methods for measuring light intensity, which have been 
at all satisfactory, are based upon the fact that silver salts blacken in the light. 
The first photographic method was proposed by Bunsen and Roscoe in 1862; 
this has been taken up by Wiesner and variously modified. After consider- 
able experiment by the writer, however, it seemed desirable to abandon all 
methods which require the use of "normal paper" and "normal black" and to 
develop a simpler one. As space is lacking for a satisfactory discussion of 
the Bunsen-Roscoe-Wiesner methods, the reader is referred to the works 
cited below. 1 Simple photometers for making light readings simultaneously 

1 Bunsen, R., and Roscoe, H. Photometrische Untersuchungen. PoggendorfFs 
Annalen., 117:529. 1862. 
Wiesner, J. 

Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. 
Wiss. Wien., I, 1893. 11,1895. 



LIGHT 



49 



or in series were constructed in 1900, and have been in constant use since 
that time. An automatic instrument capable of making accurate continuous 
records proved to be a more difficult problem. A sunshine recorder was ulti- 
mately found which yields valuable results, and very recently a recording 
photometer which promises to be perfectly satisfactory has been devised. 
Since the hourly and daily variations of sunlight in the same habitat are 
relatively small, automatic photometers are perhaps a convenience rather than 
a necessity. 

The Photometer 



77. Construction. The simple form of photometer shown in the illustra- 
tion is a light-tight metal box with a central wheel upon which a strip of 




Fig. 11. Photometer, showing front and side view. 

photographic paper is fastened. This wheel is revolved by the thumb- 
screw past an opening 6 mm. square which is closed by means of a slide 
working closely between two flanges. At the edge of the opening, and 
beneath the slide is a hollow for the reception of a permanent light stand- 
ard. The disk of the thumbscrew is graduated into twenty-five parts, 
and these are numbered. A line just beneath -the opening coincides with 

Untersuchungen iiber das photochemische Klima von Wien, Cairo, und Buitenzorg 

(Java) Denksch. Kais. Akad. Wien., 6.4. 1896. 
Untersuchungen iiber den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. 

Kais. Akad. Wien., 109. 1900. 



50 THE HABITAT 

the successive lines on the disk, and indicates the number of the exposure. 
The wheel contains twenty-five hollows in which the click works, thus mov- 
ing each exposure just beyond the opening. The metal case is made in 
two parts, so that the bottom may be readily removed, and the photographic 
strip placed in position. The water-photometer is similar except that 
the opening is always covered with a transparent strip and the whole in- 
strument is water-tight. These instruments have been made especially for 
measuring light by the C. H. Stoelting Co., 31 W. Randolph street, 
Chicago, 111. The price is $5. 

78. Filling the photometer. The photographic paper called "solio" which 
is made by the Eastman Kodak Company, Rochester, N. Y., has proved 
to be much the best for photometric readings. The most convenient size is 
that of the 8 x 10 inch sheet, which can be obtained at any supply house 
in packages of a dozen sheets for 60 cents. New "emulsions," i. e., new 
lots of paper, are received by the dealers every week, but each emulsion can 
be preserved for three to six months without harm if kept in a cool, light- 
tight place. Furthermore, all emulsions are made in exactly the same way, 
and it has been impossible to detect any difference in them. To fill the 
photometer, a strip exactly 6 mm. wide is cut lengthwise from the 8 x 10 
sheet. This must be done in the dark room, or at night in very weak light. 
The strip is placed on the wheel, extreme care being taken not to touch the 
coated surface, and fixed in position by forcing the free ends into the slit 
of the wheel by a piece of cork 8-9 mm. long. The wheel is replaced in 
the case, turned until the zero is opposite the index line, and the instrument 
is ready for use. 

79. Making readings. An exposure is made by moving the slide quickly 
in such a way as to uncover the entire opening, and the standard if the 
exposure is to be very short. Care must be taken not to pull the slide en- 
tirely out of the groove, as it will be impossible to replace it with sufficient 
quickness. The time of exposure can be determined by any watch after a 
little practice. It is somewhat awkward for one person to manage the slide 
properly when his attention is fixed upon a second hand. This is obviated 
by having one observer handle the watch and another the photometer, but 
here the reaction time is a source of considerable error. The most satis- 
factory method is to use a stop-watch. This can be held in the left hand 
and started and stopped by the index finger. The photometer is held against 
it in the right hand in such a way that the two movements of stopping the 
watch and closing the slide may be made at the same instant. The length 
of exposure is that necessary to bring the tint of the paper to that of the 



LIGHT 



51 



standard beside it. A second method which is equally advantageous and 
sometimes preferable does away with the permanent standard in the field 
and the need for a stop-watch. In this event, the strip is exposed until a 
medium color is obtained, since very light or very deep prints are harder to 
match. This is later compared with the multiple standard. In both cases, 
the date, time of day, station, number of instrument and of exposure, and 
the length of the latter in seconds are carefully noted. The instrument is 
held with the edge toward the south at the level to be read, and the open- 
ing uppermost in the usual position of the leaf. When special readings are 
desired, as for isophotic leaves, reflected light, etc., the position is naturally 
changed to correspond. In practice, it is made an invariable rule to move 
the strip for the next exposure as soon as the slide is closed. Otherwise 




Fig. 12. Dawson-Lander sun recorder. 

double exposures are liable to occur. When a strip is completely exposed 
it is removed in the dark, and a new one put in place. The former is care- 
fully labeled and dated on the back, and put away in a light-tight box in a 
cool place. 



80. The Dawson-Lander sun recorder. "The instrument consists of a 
small outer cylinder of copper which revolves with the sun, and through 
the side of which is cut a narrow slit to allow the sunshine to impinge on 
a strip of sensitive paper, wound round a drum which fits closely inside the 
outer cylinder, but is held by a pin so that it can not rotate. By means of 



52 THE HABITAT 

a screw fixed to the lid of the outer cylinder, the drum holding the sensi- 
tive paper is made to travel endwise down the outer tube, one-eighth of an 
inch daily, so that a fresh portion of the sensitive surface is brought into 
position to receive the record." The instrument is driven by an eight-day 
clock placed in the base below the drum. The slit is covered by means of 
a flattened funnel-shaped hood, and the photographic strip is protected 
from rain by a perfectly transparent sheet of celluloid. The detailed structure 
of the instrument is shown in figure 12. This instrument may be obtained 
from Lander and Smith, Canterbury, England, for $35. 

In setting up the sunshine recorder, the axis should be placed in such a 
position that the angle which it makes with the base is the same as the 
altitude of the place where the observations are made. This is readily done 
by loosening the bolts at either side. The drum is removed, the celluloid 
sheet unwound by means of the key which holds it in place, the sensitive 
strip put in position, and the sheet again wound up. Strips of a special sensi- 
tive paper upon which the hours are indicated are furnished by the makers 
of the instrument, but it has been found preferable to use solio strips in 
order to facilitate comparison with the standards. The drum is placed on 
the axis, and is screwed up until it just escapes the collar at the top of the 
spiral. The clock is wound and started, and the outer cylinder put on so 
that the proper hour mark coincides with the index on the front of the base. 

As a sunshine recorder, the instrument gives a perfect record, in which 
the varying intensities are readily recognizable. Since the cylinder moves 
one-half inch in an hour, and the slit is .01 of an inch, the time of each 
exposure is 72 seconds. This gives a very deep color on the solio paper, 
which results in a serious error in making comparisons with the standard. 
On account of the hood, diffuse light is not recorded when it is too weak 
to cast a distinct shadow. It seems probable that this difficulty will be over- 
come by the use of a flat disk containing the proper slit, and in this event 
the instrument will become of especial value for measuring the diffuse light 
of layered formations. The celluloid sheet constitutes a source of error in 
sunlight on account of the reflection which it causes. This can be prevented 
by using the instrument only on sunny days, when the protection of the 
sheet can be dispensed with. 

81. The selagraph. This instrument is at present under construction, 
and can only be described in a general way. In principle it is a simple 
photometer operating automatically. It consists of a ligiit-tight box pref- 
erably of metal, which contains an eight-day lever clock. Attached to 
the arbor of the latter is a disk 7 inches in diameter bearing on its circum- 
ference a solio strip 1 cm. wide and 59 cm. long. The opening in the box 



LIGHT 53 

for exposure is 6 mm. square and is controlled by a photographic shutter. 
The latter is constructed so that it may be set for 5, 10, or 20 seconds, since 
a single period of exposure can not serve for both sun and shade. The 
shutter is tripped once every two hours, by means of a special wheel revolv- 
ing once a day. Each exposure is 6 mm. square, and is separated by a 
small space from the next one. Twelve exposures are made every 24 hours, 
and 84 during the week, though, naturally, the daytime exposures alone are 
recorded. Comparisons with the multiple standard are made exactly as in 
the case of the simple photometer. The selagraph is made by the C. H. 
Stoelting Co., Chicago, Illinois. 

Standards 

82. Use. The light value of each exposure is determined by reference to 
a standard. When the photometer carries a permanent standard, each ex- 
posure is brought to the tint of the latter, and its value is indicated by the 
time ratio between them. Thus, if the standard is the result of a 5-second 
exposure to full sunlight at meridian, and a reading which corresponds in 
color requires 100 seconds in the habitat concerned, the light of the latter 
is twenty times weaker or more diffuse. Usually, the standard is regarded 
as unity, and light values figured with reference to it, as .05. With the 
sciagraph such a use of the standard is impossible, and often, also, with the 
photometer it is unnecessary or not desirable. The value of each exposure 
in such case is obtained by matching it with a multiple standard, after the 
entire strip has been exposed. The further steps are those already indicated. 
After the exact tint in the standard has been found, the length of the 
reading in seconds is divided by the time of the proper standard, and the 
result expressed as above. 

83. Making a standard. Standards are obtained by exposing the photo- 
meter at meridian on a typically clear day, and in the field where there is 
the least dust and smoke. Exception to> the latter may be made, of course, 
in obtaining standards for plant houses located in cities, though it is far 
better to have the same one for both field and control experiment. Usable 
standards can be obtained on any bright day at the base station. Indeed, valu- 
able results are often secured by immediate successive sun and shade read- 
ings in adjacent habitats, where the sun reading series is the sole standard. 
Preferably, standards should be made at the solstices or equinoxes, and at a 
representative station. The June solstice is much to be preferred, as it 
represents the maximum light values of the year. Lincoln has been taken 
as the base station for the plains and mountains. It is desirable, however, 



54 THE HABITAT 

that a national or international station be ultimately selected for this pur- 
pose, in order that light values taken in different parts of the world may be 
readily compared. 

84.- Kinds of standards.. The base standard is the one taken at Lincoln 
(latitude 41 ° N.) at meridian June 20-22. This is properly the unit to which 
all exposures are referred, but it has been found convenient to employ the 
Minnehaha standard as the base for the Colorado mountains, in order to 
avoid reducing each time. ' Relative standards are frequently used for tem- 
porary purposes. Thus, in comparing the light intensities of a series of 
formations, one to five standards are exposed on the solio strip before be- 
ginning the series of readings. Proof standards are the exposed solio strips, 
which fade in the light, and can, in consequence, be kept only a few weeks 
without possibility of error. The fading can be prevented by "toning" the 
strip, but in this event the exposures must be fixed in like manner before 
they can be compared. This process is inconvenient and time-consuming. It 
is also open to considerable error, as the time of treatment, strength of solu- 
tion, etc., must be exactly equivalent in all instances. Permanent standards 
are accurate water-color copies of the originals obtained by the photometer. 
These have the apparent disadvantage of requiring a double comparison or 
matching, but after a little practice it is possible to reproduce the solio tints 
so that the copy is practically indistinguishable from the original. The most 
satisfactory method is to make a long stroke of color on a pure white paper, 
since a broad wash is not quite homogeneous, and then to reject such parts 
of the stroke as do not match exactly. Permanent standards fade after a 
few month's use, and must be replaced by parts of the original stroke. 
Single standards are made by one exposure, while multiple ones have a 
series of exposures filling a whole light strip. These are regularly obtained 
by making the exposures from 1-10 seconds respectively, and then increas- 
ing the length of each successive exposure by 2 seconds. Single exposures 
of 1-5 seconds as desired usually serve as the basis for permanent stand- 
ards, but a multiple standard may also be copied in permanent form. Ex- 
posures for securing standards must be made only under the most favorable 
conditions, and the length in seconds must be exact. The use of the stop- 
watch is imperative, except where access may be had to an astronomical 
clock with a large second hand, which is even more satisfactory. The 
length of time necessary for the series desired is reckoned beforehand, and 
the exposures begun so that the meridian falls in the middle of the process. 

Single standards are exceedingly convenient in photometer readings, but 
they are open to one objection. In the sunshine it is necessary to make in- 
stant decision upon the accuracy of the match, or the exposure becomes too 



LIGHT 



55 



deep. In the shade where the action is slower, this difficulty is not felt. 
For this reason it is usually desirable to check the results by a multiple stand- 
ard, and in the case of sciagraph records, where the various exposures show 
a wide range of tint, light values are obtainable only by direct comparison 
with the multiple standard. The exact matching of exposure and standard 
requires great accuracy, but with a little practice this may be done with 
slight chance of error by merely moving the exposure along the various 
tints of the standard until the proper shade is found. The requisite skill is 
soon acquired by running over a strip of exposures several times until the 
comparisons always yield the same results for each. The margin of error 
is practically negligible when the same person makes all the comparisons, 
and in the case of two or three working on the same reading the results 
diverge little or not at all. The efficient difference for light is much more 
of a variable than is the case with water-content. It has been determined 
so far only for a few species, all of which seem to indicate that appreciable 
modification in the form or structure of a leaf does not occur until the 
reduction in intensity reaches .1 of the meridian sunlight at the June sol- 
stice. The error of comparison is far less than this, and consequently may 
be ignored, even in the most painstaking inquiry. 

Readings 

85. Time. The intensity of the lig-ht incident upon a habitat varies peri- 
odically with the hour and the day, and changes in accord with the changing 
conditions of the sky. The light variations on cloudy days can only be de- 
termined by the photometer. While these can not be ignored, proper com- 
parisons can be instituted only between the readings taken on normal days 
of sunshine. The sunlight varies with the altitude of the sun, i. e., the angle 
which its rays make with the surface at a given latitude. This angle reaches 
a daily maximum at meridian. The yearly maximum falls on June 22, and 
the angle decreases in both directions through the year to a minimum on 
December 22. At equal distances from either solstice, the angle is the same, 
e. g., on March 21 and September 23. At Lincoln (41 ° N. latitude) the 
extremes at meridian are 73 ° and 26 ; at Minnehaha (39°) they are 75 
and 28 °. The extremes for any latitude may be found by subtracting 
its distance in degrees north of the two tropics from 90. Thus, the 50th 
parallel is 26. 5 ° north of the tropic of Cancer, and the maximum altitude of 
the sun at a place upon it is 63.5 °. It is 73.5 ° north of the tropic of Capri- 
corn, and the minimum meridional altitude is 16.5 °. 

The changes in the amount of light due to the altitude of the sun are pro- 
duced by the earth's atmosphere. The absorption of light rays is greatest 
near the horizon, where their pathway through the atmosphere is longest, 



56 THE HABITAT 

and it is least at the zenith. The absorption, and, consequently, the relative 
intensity of sunlight, can be determined at a given place for each hour of 
any sunshiny day by the use of chart 13. This chart has been constructed 
for Lincoln, and will serve for all places within a few degrees of the 40th 
parallel. The curves which show the altitude of the sun at the various times 
of the day and the year have been constructed by measurements upon the 
celestial globe. Each interval between the horizontal lines represents 2 de- 
grees of the sun's altitude. The vertical lines indicate time before or after 
the apparent noon, the intervals corresponding to 10 minutes. If the rela- 
tive intensity at Lincoln on March 12 at 3:00 p.m. is desired, the apparent 
noon for this day must first be determined. A glance at the table shows that 
the sun crosses the meridian on this day at 9 minutes 53 seconds past noon at 
the 90th meridian. The apparent noon at Lincoln is found by adding 26 
minutes 49 seconds, the difference in time between Lincoln and a point on 
the 90th meridian. When the sun is fast, the proper number of minutes is 
taken from 26 minutes 49 seconds. The apparent noon on March 12 is thus 
found to fall at 12:37 p - M v and 3:00 p.m. is 2 hours and 23 minutes later. 
The sun's altitude is accordingly 36 °. If the intensity of the light which 
reaches the earth's surface when the sun is at zenith is taken as 1, the table of 
the sun's altitudes gives the intensity at 3 :oo p.m. on March 12 as .85. 

For places with a latitude differing by several deg'rees from that of Lin- 
coln, it is necessary to construct a new table of altitude curves from the 
celestial globe. It is quite possible to make a close approximation of this 
from the table given, since the maximum and minimum meridional altitude, 
and hence the corresponding light intensity, can be obtained as indicated 
above. For Minnehaha, which is on the 105th meridian, and for other places 
on standard meridians, i. e., 6o°, 75 , 90 , and 120 W., the table of apparent 
noon indicates the number of minutes to be added to 12 noon, standard time, 
when the sun is slow, and to be subtracted when the sun is fast. The time at 
a place east or west of a standard meridian is respectively faster or slower 
than the latter. The exact difference in minutes is obtained from the dif- 
ference in longitude by the equation, i5°=:i hour. Thus, Lincoln, 96 ° 42' 
W. is 6° 42' west of the standard meridian of 90° ; it is consequently 26 min- 
utes 49 seconds slower, and this time must always be added to the apparent 
noon as determined from the chart. At a place east of a standard meridian, 
the time difference is, of course, subtracted. 

The actual differences in the light intensity from hour to hour and day to 
day,' which are caused by variations in the sun's altitude, are not as great 
as might be expected. For example, the maximum intensity at Lincoln, 
June 22, is .98 ; the minimum meridional intensity December 22 is .73. The 
extremes on June 22 are .98 and .33 (the latter at 6:00 a.m. and 6:00 p.m. 



LIGHT 



57 



approximately) ; between 8:00 a.m. and 4:00 p.m. the range in intensity is 
from .90 to .98 merely. On December 22, the greatest intensity is .52, the 
least .20 (the latter at 8 :oo a.m. and 4 :oo p.m. approximately). If the grow- 
ing season be taken as beginning with the 1st of March and closing the 1st of 
October, the greatest variation in light intensity at Lincoln within a period of 
10 hours with the meridian at its center (cloudy days excepted) is from .33 to 
.98. In a period of 8 hours, the extremes are .65 to .98, i. e., the greatest 
variation, .3, is far within the efficient difference, which has been put at .9. 



ait 



Q\h\u$t 


Intensity..' 


90° 


1.000 


to] 


m 


70' 


.ft* 


b<f 


. ?i? 


€(f 


. 932 


vo- 


■ 909 


30' 


. 7*1 


to' 


. k«2 


/ 0' 


*3t 




Fig. 13. Chart for the determination of the sun's altitude, and the corresponding light intensity. 



For the growing period, then, readings made between 8 :oo a.m. and 4 :oo p.m. 
on normal sunshiny days may be compared directly, without taking into ac- 
count the compensation for the sun's altitude. Until the efficient difference 
has been determined for a large number of species, however, it seems wise to 
err on the safe side and to compensate for great differences in time of day 
or year. In all doubtful cases, the intensity obtained by the astronomical 






58 



THE HABITAT 



method should also be checked by photometric readings. A slight error 
probably enters in, due to reflection from the surface of the paper, and to 
temperature, but this is negligible. 

86. Table for determining apparent noon 



DATE 


TIME LI> 
EQUATION N 


rCOLN 
OON 


DATE 


TIME LINCOLN 
EQUATION NOON 






Sun slow: -\- 26r 


a. 49s. 




Sun slow:-\- 26n 


i. 49s. 


January 


1.. 


3m. 47s. 12 


:31 p.m. 


July 5.. 


4m. 19s. 12 


:31 p.m. 


<< 


6.. 


6 7 


:33 


10.. 


5 7 


:32 


«« 


11.. 


8 12 


:35 


20.. 


6 6 


:33 


«< 


16.. 


10 3 


:37 


August 4. . 


5 53 


:33 


<< 


21.. 


11 35 


:38 


14.. 


4 30 


:31 


<< 


26.. 


12 48 


:40 


19.. 


. 3 28 


:30 


<< 


31.. 


13 41 


:40 


24.. 


2 13 


:29 


February 10. . 


14 27 


:41 


29.. 


48 


.28 


« 


20.. 


13 56 


.41 




Sun fast: — 




March 


2. 


12 18 


39 


September3. . 


45 


•26 


« 


7.. 


11 10 


38 


8.. 


2 25 


24 


(< 


12.. 


9 53 


37 


13.. 


4 9 


23 


« 


17.. 


• 8 29 


36 


18.. 


5 55 


21 


<< 


22.. 


6 59 


34 


23.. 


7 41 


19 


<( 


27.. 


5 27 


32 


28.. 


9 23 


17 


April 


1.. 


•3 55 


31 


October 3.. 


10 59 


16 


<< 


6.. 


2 27 


29 


8.. 


12 26 


14 


« 


11.. 


1 3 


28 


13: 


13 43 


13 






Sun fast: — 




18.. 


14 48 


12 


« 


16.. 


13 


27 


23.. 


15 37 


11 


« 


21.. 


1 20 


25 


November 2. . 


16 20 ; 


10 


<( 


26.. 


2 16 


24 


12.. 


15 45 : 


11 


May 


1.. 


3 


24 


17.. 


14 54 : 


12 


<< 


16.. 


3 48 


23 


22.. 


13 44 


13 


<< 


31.. 


2 33 : 


24 


27.. 


12 14 : 


15 


June 


5.. 


1 45 : 


25 


December 2. . 


10 25 : 


16 


<« 


10.. 


49 : 


26 


7.. 


8 21 : 


18 






Sun slow: -\- 




12.. 


6 5 : 


21 


<( 


15.. 


13 : 


27 


17.. 


3 41 : 


23 


<« 


20.. 


1 18 : 


28 


" 22.. 


1 12 : 


26 


<« 


25.. 


2 22 : 


29 




Sun slow: -4- 




(i 


30.. 


3 22 : 


30 


27.. 


1 17 : 


28 



LIGHT 59 

87. Place. The effect of latitude upon the sun's altitude, and the conse- 
quent light intensity have been discussed in the pages which precede. Lati- 
tude has also a profound influence upon the duration of daylight, but the 
importance of the latter apart from intensity is not altogether clear. The 
variation of intensity due to altitude has been greatly overestimated; it is 
practically certain, for example, that the dwarf habit of alpine plants is not 
to be ascribed to intense illumination, since the latter increases but slightly 
with the altitude. It has been demonstrated astronomically that about 20 
per cent of a vertical ray of sunlight is absorbed by the atmosphere 
by the time it reaches sea level. At the summit of Pike's Peak, which is 
14,000 feet (4,267 meters) high, the barometric pressure is 17 inches, and 
the absorption is approximately 11 per cent. In other words, the light at 
sea level is 80 per cent of that which enters the earth's atmosphere; on the 
summit of Pike's Peak it is 89 per cent. As the effect of the sun's altitude 
is the same in both places, the table of curves on page 57 will apply to 
both. Taking into account the difference in absorption, the maximum in- 
tensity at sea level and at 14,000 feet on the fortieth parallel is .98 and 1.09 
respectively. The minimum intensities between 8:00 a.m. and 4:00 p.m. of 
the growing period are .64 and .71 respectively. The correctness of these 
figures has been demonstrated by photometer readings, which have given al- 
most exactly the same results. Such slight variations are quite insufficient 
to produce an appreciable adjustment, particularly in structure. They are 
far within the efficient difference, and Reinke 1 has found, moreover, that 
photosynthetic activity in Elodea is not increased beyond the normal in 
sunlight sixty times concentrated. In consequence, it is entirely unneces- 
sary to take account of different altitudes in obtaining light values. 

The slope of a habitat exerts a considerable effect upon the intensity of 
the incident light. If the angle between the slope and the sun's ray be 90°, 
a square meter of surface will receive the maximum intensity, I. At an 
angle of io°, the same area receives but .17 of the light. This relation be- 
tween angle and intensity is show T n in the table which follows. The influence 
of the light, however, is felt by the leaf, not by the slope. Since there is 
no connection between the position of the leaf and the slope of the habitat, 
the latter may be ignored. In consequence, it is unnecessary to make al- 
lowances for the direction of a slope, viz., whether north, east, south, or 
west, in so far as light values are concerned. The angle which a leaf makes 
with its stem determines the angle of incidence, and hence the amount of 
light received by the leaf surface. This is relatively unimportant for two 
reasons. This angle changes hourly and daily with the altitude of the 

1 Reinke, J. Bot. Zeit., 41:713. 1883. 



6o THE HABITAT 

sun, and the intensity constantly swings from one extreme to the other. 
Moreover, the extremes i.oo and 0.17, even if constant, are hardly sufficient 
to produce a measurable result. When the angle of the leaf approaches 90 °, 
there is the well-known differentiation of leaf surfaces and of chlorenchym, 
but this has no relation to the angle of incidence. 

Table of Intensity at Various Angles 



S T GLE 


INTENSITY 


90 


1.00 


80 


.98 


70 


.94 


60 


.87 


50 


.77 


40 


.64 


30 


.50 


20 


.34 


10 


.17 



In the sunlight, it makes no difference at what height a light reading is 
taken. In forest and thicket as well as in some herbaceous formations, the 
intensity of the light, if there is any difference, is greatest just beneath the 
foliage of the facies. In forests especially, the light is increasingly diffuse 
toward the ground, particularly where layers intervene. In woodland for- 
mations, moreover, the exact spot in which a reading is made must be care- 
fully chosen, unless the foliage is so dense that the shade is uniform. A 
very satisfactory plan is to take readings in two or more spots where the 
shade appears to be typical, and to make a check reading in a "sunfleck," 
a spot where sunlight shows through. In forests and thickets, the sunflecks 
are fleeting, and the light value is practically that of the shade. In pass- 
ing into open woodland and thicket, the sunflecks increase in size and per- 
manence, until finally they exceed the shade areas in amount and become 
typical of the formation. 

Reflected and Absorbed Light 

88. The fate of incident light. The light present in a habitat and incident 
upon a leaf is not all available for photosynthesis. Part is reflected or 
screened out by the epidermis, and a certain amount passes through the 
chlorenchym, except in very thick leaves. The light absorbed is by far the 
greatest in the majority of species. Many plants with dense coatings of 
hairs reflect or withhold more light than they absorb, and the amount of 
light reflected by a thick cuticule is likewise great. As light is imponder- 
able, the actual amount absorbed or reflected by the leaf can not be deter- 
mined. It is possible, however, to express this. in terms of the total amount 



LIGHT 



6l 



received, by means of readings with solio paper, and the knowledge thus 
obtained is of great importance in interpreting the modifications of certain 
types of leaves. For example, a leaf with a densely hairy epidermis may 
receive light of the full intensity, I ; the amount reflected or screened out by 
the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and 
that transmitted, nil. In the majority of cases, however, the absorbed light 
is considerably more than the 
amount reflected 



or trans- 



mitted. 




89. Methods of determina= 
tion. If results are to be of 
value, reflected and transmit- 
ted light must be determined 
in the habitat of the plant 
simultaneously with the total 
light which a leaf receives. 
An approximation of the 
light reflected from a leaf 
surface is secured by placing 
the photometer so that the 
light reflected is thrown upon 
the solio strip. A much more 
satisfactory method, however, 
is to determine it. in connec- 
tion with the amount of light 
transmitted through the epi- 
dermis. This is done by 
stripping a piece of epidermis 
from the upper surface of the 
leaf and placing it over the 
slit in the photometer for an 
exposure. An exposure in 
the full light of the habitat is made simultaneously with another photometer, 
or immediately afterward upon the same strip. When the epidermis is 
not too dense, both exposures are permitted to reach the same tint, and the 
relation between them is precisely that of their lengths of exposure. Or- 
dinarily the two exposures are made absolutely simultaneous by placing 
the epidermis over half of the opening, leaving the other half to record the 
full light value, and the results, or epidermis prints, are referred to a multi- 



Fig. 14. Leaf print: exposed 10 m., 11 a.m. Au- 
gust 20. The leaves are from sun and shade 
forms of Bursa bursa-pastoris, Rosa say it, 
Thalictrum sparsiflorum, and Machaer anther a 
aspera. In each the shade leaf prints more 
deeply. 



62 



THE HABITAT 



pie standard. The difference between the two values thus obtained repre- 
sents the amount of reflected light together with that screened by the epi- 
dermis. The amount of light transmitted through the leaf may be measured 
in the same way by using the leaf itself in place of the epidermis alone. 
The time of exposure is necessarily long, however, and it has been found 
practicable to obtain leaf prints by exposing the leaf in a printing frame, 
upon solio paper, at the same time that the epidermis print is made. In a 
few species both the upper and lower epidermis can be removed and the 
amount of light absorbed determined directly by exposing the strip covered 

with the chlorenchym. Generally, 
however, this must be computed by 
subtracting the sum of the per cents 
of reflected and transmitted light 
from too per cent, which represents 
the total light. 

90. Leaf and epidermis prints. In 

diphotic leaves the screening effect 
of the lower epidermis may be 
ignored. Isophotic sun leaves, i. e., 
those nearly upright in position or 
found above light-colored, reflecting 
soils, are usually strongly illumi- 
nated on both sides, and the ab- 
sorbed light can be obtained only by 
measuring the screening effect of 
both epiderms. Shade leaves and 
submerged leaves often contain 
chloroplasts in the epidermis, and the 
above method can not be applied to 
them. In fact, in habitats where the 
light is quite diffuse, practically all 
incident light is absorbed. The rare 
exceptions are those shade leaves with a distinct bloom. In addition to 
their use in obtaining the amount of light absorbed, both leaf and epidermis 
prints are extremely interesting for the direct comparison of light relations 
in the leaves of species belonging to different habitats. The relative screen- 
ing value of the upper and lower epidermis, or of the corresponding epiderms 
of two ecads or two species, is readily ascertained by exposing the two side 
by side in sunshine, over the slit in the photometer. For leaf prints fresh 
leaves are desirable, though nearly the same results can be obtained from 




Fig. 15. Leaf print: exposure as before. 
Sun and shade leaves of Achillea lanulosa, 
Capnoides aureum, Antennaria umbri- 
nella, Galium boreale, and Polentilla 
propinqua. 



LIGHT 



63 



leaves dried under pressure. The leaves are grouped as desired on the glass 
of a printing frame, and covered with a sheet of solio. They are then ex- 
posed to full sunlight, preferably at meridian, and the prints evaluated by 
means of the multiple standard. This method is especially useful in the 
comparison of ecads of one species. These differences due to transmitted 
light are very graphic, and can easily be preserved by "toning" the print 
in the usual way. 

Expression of Results 

91. Light records. The actual photographic records obtained by photo- 
meter and selagraph can at most be kept but a few months, unless they are 
"toned" or fixed. "Toning" modifies the color of the exposure materially, 
and changes its intensity so that it can not be compared with readings not 
fixed. It would involve a great deal of inconvenience to make all compari- 
sons by means of toned strips and standard, even if it were not for the fact 
that it is practically impossible to obtain exactly the same shade in lots 
toned at different times. The. field record, if carefully and neatly made, 
may well take the place of a permanent one. The form is the following: 



Q 


U 

O 






03 


'd 

•3 

1—1 

< 


V 

u 

m 
O 

a 

W 




u 

O 




d 




'd 
u 
ctf 

a 


bCcS 

3> 


PQ 


T3 
V 


a 

in -w 


V 
.O 

2 W 


14/9/04 


12:C0 m. 
12:05 p.m. 
12:15 p.m. 


Spruce 
Spruce 
Brook b'nk 


Milky Way- 
Moss Glen 
Grotto 


2600 m. 
2500 m. 
2500 m. 


N.K.£0° 
I<evel 
E. 3° 


Opulaster 

Streptopus 

Filix 


lfoot 
Surface 


2:10 
2:12 
2:13 


ieos. 

240 s. 
360 s. 


3s 

3 s. 
3 s. 


.019 
.012 
.008 










4 1 










u 





















92. Light sums, means, and curves. Owing to the fact that the sela- 
graph has not yet been used in the field, no endeavor has been made to de- 
termine the light value for every hour of the day in different habitats. 
Consequently there has been no attempt to compute light sums and means. 
Photometer readings have sufficed to interpret the effect of light in the 
structure of the formation, and of the individual, but they have not been 
sufficiently frequent for use in ascertaining sums and means. The latter 
are much less valuable than the extremes, especially when the relative dura- 
tion of these is indicated. Means, however, are readily obtained from 
the continuous records. Light sums are probably impracticable, as the 
factor is not one that can be expressed in absolute terms. The various kinds 
and combinations of light curves are essentially the same as for humidity. 
The level curve through a series of habitats is the most illuminating, but 
the day curve of hour variations is of considerable value. The curve of 



64 THE HABITAT 

daily duration, based upon full sunlight, is also of especial importance for 
plants, and stations which receive both sun and shade during the day. 

TEMPERATURE 

93. In consequence of its indirect action, temperature does not have a 
striking effect upon the form and structure of the plant, as is the case 
with water and light. Notwithstanding, it is a factor of fundamental im- 
portance. This is especially evident in the character and distribution of 
vegetation. It is also seen in the germination and growth of plants, in the 
length of season, and in the important influence of temperature upon hu- 
midity, and hence upon water-content. Because of its intimate relation with 
the comfort of mankind, the determination of temperature values has re- 
ceived more attention than that of any other factor, and excellent simple 
and recording instruments are numerous. For plants, it is also necessary 
to employ instruments for measuring soil temperatures. The latter un- 
questionably have much less meaning for the plant than the temperatures 
of the air, but they have a direct influence upon the imbibition of water, 
and upon germination. 

Thermometers 

94. Air thermometers. The accurate measurement of temperature re- 
quires standard thermometers. Reasonably accurate instruments may be 
standardized by determining their error, but they are extremely unsatis- 
factory in practice, since they result in a serious waste of time. Accurate 
thermometers which read to the degree are entirely serviceable as a rule, 
but instruments which read to a fraction of a degree are often very much 
to be desired. The writer has found the "cylindrical bulb thermometer, 
Centigrade scale" of H. J. Green, to be an exceedingly satisfactory instru- 
ment. The best numbers for general use are 247 and 251, which read from 
-1 5° to 50 C. and are graduated in .2°. They are respectively 9 and 12 
inches long, and cost $2.75 and $3.50. These instruments are delicate and 
require careful handling, but even in class work this has proved to be an 
advantage rather than otherwise. In making readings of air temperatures 
with such thermometers, constant precautions must be taken to expose the 
bulb directly to the wind and to keep it away from the hand and person. 

95. Soil thermometers. The thermometer described above has been used 
extensively for soil temperatures. The determination of the latter is con- 
veniently combined with the taking of soil samples, by using the hole for 
a temperature reading. When carefully covered, these holes can be used 
from day to day throughout the season without appreciable error, even in 



TEMPERATURE 



65 



gravel soils. Repeated tests of this have been made by simultaneous read- 
ings in permanent and newly made holes, and the results have always been 
the same. It has even been found that the error is usually less than 1 degree 
when the hole is left uncovered, if it is more than 9 inches deep. A slight 
source of error lies in the fact that the thermometer must be raised to make 
the reading. With a little practice, however, the top of the 
column of mercury may be raised to the surface and read be- 
fore the change of temperature can react upon it. This is 
especially important in very moist or wet soils where the bulb 
becomes coated with a film of moisture. This evaporates 
when the bulb is brought into the air, and after a moment or 
two the mercury slowly falls. 

Regular soil thermometers are indispensable when read- 
ings are desired at depths greater than 12-18 inches. They 
possess several disadvantages which restrict their use almost 
wholly to permanent stations. It is scarcely possible to carry 
them on field trips, and the time required to place them in 
the soil renders them practically useless for single readings. 
Moreover, the instruments are expensive, ranging in price 
from $7 for the two-foot thermometer, to $19 for the eight- 
foot one. W Tien it is recognized that deep-seated tempera- 
tures are extremely constant and that the slight fluctuations 
affect, as a rule, only the relatively stable shrubs and trees, 
it is evident that such temperatures are of restricted impor- 
tance. Still, in any habitat, they must be ascertained before 
they can well be ignored, though it is unwise to spend much 
time and energy in their determination. Soil thermometers 
of the form illustrated may be obtained from H. J. Green, 
Brooklyn. 

96. Maximum=minimum thermometers. These are used 
for determining the range of temperature within a given 
period, usually a day. Since they are much cheaper than 
thermographs, they can replace these in part, although they 
merely indicate the maximum and minimum temperatures for 
the day, and do not register the time when each occurs. The 
maximum is a mercurial thermometer with a constriction in the tube just 
above the bulb ; this allows the mercury to pass out as it expands, but 
prevents it from running back, thus registering the maximum temperature. 
The minimum thermometer contains alcohol. The column carries a tiny 
dumbbell-shaped marker which moves down with it, but will not rise as 



Fig. 16. Soil 
thermometer 



66 



THE HABITAT 




the liquid expands. This is due to the fact that the fluid expands too 
slowly to carry the marker upward, while the surface tension causes it to 
be drawn downward as the fluid contracts. The minimum temperature 
is indicated by the upper end of the marker. In setting up the thermome- 
ters, they are attached by special thumbscrews to a support which holds 
them in an oblique position. The minimum is placed in a special holder 

above the maxi- 
mum which rests 
on a pin that is 
used also for 
screwing the piv- 
ot-screw into po- 
sition. The sup- 
Fig. 17. Maximum-minimum thermometer. pQrt [s screwed 

tightly to the cross-piece of a post, or in forest formations it is fastened di- 
rectly to a board nailed upon a tree trunk. A shelter has not been used in 
ecological work, although it is the rule in meteorological observations. The 
minimum thermometer is set for registering by raising the free end, so 
that the marker runs to the end of the column. The mercury of the maxi- 
mum is driven back into the bulb by whirling it rapidly on the pivot-screw 
after the pin has been taken out. This must be done with care in order 
that the bulb may not be 
broken. As soon as the in- 
strument comes to rest, it 
is raised and the pin re- 
placed, great care being 
taken to lift it no higher 
than is necessary. When 
the night maximum is 
sought, the thermometer 
should be whirled several 
times in order to drive the 
column sufficiently low. 
Usually, in such cases, a record is made of this point to make sure that 
the maximum read is the actual one. If the pivot-screw is kept well oiled, 
less force will be required to drive the mercury back. In practice, the 
thermometers have been observed at 6:00 a.m. and 6:00 p.m. each day, 
thus permitting the reading- of the maximum-minimum for both day and 
night. Pairs of maximum-minimum thermometers are to be obtained from 
H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, Baltimore, 
Maryland, at a cost of $8.25. 




Fig. 18. Terrestrial radiation thermometer. 



TEMPERATURE 



67 



97. Radiation thermometers. These are used to determine the radiation 
in the air, and from the soil, i. e., for solar and terrestrial radiation. The 
latter alone has been employed in the study of habitats, chiefly for the 
purpose of ascertaining the difference in the cooling of different soils at 
night. The terrestrial radiation thermometer is merely a special form of 
minimum thermometer, so 

arranged in a support that 
the bulb can be placed di- 
rectly above the soil or plant 
to be studied. It is other- 
wise operated exactly like 
the minimum thermometer, 
and the reading gives the 
minimum temperature which 
the air above the plant or 
soil reaches, not the amount 
of radiation. As a conse- 
quence, these instruments 
are valuable only where 
read in connection with a 
pair of maximum-minimum 
thermometers in the air, or 
when read in a series of in- 
struments placed above dif- 
ferent soils or plants. 

98. Thermographs. Two 

types of standard instru- 
ments are in general use 
for obtaining continuous 
records of air temperatures. 

These are the Draper thermograph, made by the Draper Manufacturing 
Company, 152 Front St., New York city ($25 and $30), and the Richard 
thermograph sold by Julien P. Friez, Baltimore ($50). After careful 
trial had demonstrated that they were equally accurate, the matter of cost 
was considered decisive, and the Draper thermograph has been used ex- 
clusively in the writer's own work. This instrument closely -resembles the 
psychrograph manufactured by the same company. It is made in two sizes, 
of which the larger one is the more satisfactory on account of the greater 
distance between the lines of the recording disk. The thermometric part 
consists of two bimetallic strips, the contraction and expansion of which 




Fig. 19. Draper thermograph. 



68 



THE HABITAT 



are communicated to a hand carrying a pen. The latter traces a line on the 
record sheet which is attached to a metal disk made to revolve by an eight- 
day clock. In practice the thermograph is set up in the shelter which con- 
tains the psychrograph, and in exactly the same manner. The clock is 
wound, the record put in place, and the pen inked in the same way also. 
The proper position of the pen is determined by making a careful ther- 
mometer reading under the 
shelter, and then regulat- 
ing the pen-hand by means 
of the screws at the base 
of it. A similar test read- 
ing is also made each 
week, when the clock is re- 
wound. A record sheet 
may be left in position for 
three weeks, the pen being 
filled each week with a dif- 
ferent ink. The fixed or- 
der of using the inks is red, 
blue, and green as already 
indicated. 

Owing to the fact that 
they are practically station- 
ary, soil thermographs are 
of slight value, except at 
base stations. Here, the 
facts that they are expen- 
sive, that the soil tempera- 
tures are of relatively lit- 
tle importance, and that 
they can be determined as 
easily, or nearly so, by simple thermometers, make the use of such instru- 
ments altogether unnecessary, if not, indeed, undesirable. In a perfectly 
equipped research station, they undoubtedly have their use, but at ordinary 
stations, and in the case of private investigators, their value is in no wise 
commensurate with their cost. 




Fig. 20. Shelter for thermograph. 



TEMPERATURE 



Readings 



69 



99. Time. The hourly and daily fluctations of the temperature of the 
air render frequent readings desirable. It is this variation, indeed, which 
makes single readings, or even series of them, inconclusive, and renders 
the use of a recording instrument almost imperative in the base station at 
least. Undoubtedly, a set of simultaneous readings at different heights in 
one station, or at the same height in different stations, especially if made 
at the maximum, have much value for comparison, but their full significance 
is seen only when they are referred to a continuous base record. Such 
series, moreover, furnish good results for purposes of instruction. In re- 




Fig. 21. Richard thermograph. 

search work, however, it has been found imperative to have thermographs 
in habitats of widely different character. With these as bases, it is possible 
to eke them out with considerable satisfaction by means of maximum- 
minimum thermometers in less different habitats, or in different parts of 
the same habitat. Naturally these are less satisfactory, and are used only 
when expense sets a limit to the number of thermographs. In a careful 
analysis of a single habitat, more can be gained by one base thermograph 
supplemented by three pairs of maximum-minimum thermometers in dissimi- 
lar areas of the habitat than by two thermographs, and the cost is the 
same. 



7o 



THE HABITAT 



100. Place and height. For general air temperatures, thermograph and 
thermometer readings are made at a height of 3 feet (1 meter). Soil tem- 
peratures are regularly taken at the surface and at a depth of 1 foot. 
When a complete series of simultaneous readings is made in one station, 
the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 
to, and 15 inches in the soil. When sun and shade occur side by side in 
the same formation, as is true of many thickets and forests, surface read- 
ings are regularly made in both. Similarly, valuable results are obtained 
by making simultaneous readings on the bare soil, on dead cover, and upon 
a leaf, while the influence of cover is readily ascertained by readings upon 
it and beneath it. A full series of station readings made at the same time 
upon north, east, south, and west slopes is of great importance in studying 
the effects of exposure. 

Expression of Results 

101. Temperature records. Neither field nor permanent form is re- 
quired for thermographic records, other than the record sheet itself, which 
contains all the necessary information in a fairly convenient form. Al- 
though the temperature of a particular hour and clay can not be read at a 
mere glance, it can be obtained so easily that it is a waste of time to make 
a tabular copy of each record sheet. For thermometer readings, either sin- 
gle or in series, -the following form is used : 



Q 


u 

3 





CO 


< 


u 

O 

a, 

X 

W 


17/8/01 


6:33 a.m 


Spruce 


Jack Brook 


2553 m. 


N.K. 5° 


'' 


" 


Half gravel 


Hiawatha 


2530 m. 


N.E. 7° 


It 


6:30 p m 


■spruce 


Jack Brook 


2550 m. 


N.E. 5° 


" 


" 


Half gravel 


Hiawatha 


2550 m. 


N.E. 7° 



• 

5 >> 

s 


POSITION OF 
READING 



gbb 




3 feet 


Surf. 


12 in. 


Mertensiare 


9° 


9° 


9.8° 


10° 


Clear 


Asterare 


11.2° 


11.2° 


14.8° 


10° 


Clear 


Mertensiare 


11.4° 


11.4° 


9 8° 


11° 


Cloudy 


Asterare 


12° 


13.8° 


16.4° 


11° 


Cloudy 



102. Temperature sums and means. The amount of heat, i. e., the num- 
ber of calories received within a given time by a definite area of plant sur- 
face, can be determined by means of a calorimeter. From this the tempera- 
ture sum of a particular period may be obtained by simple addition. In 
the present condition of our knowledge, it is impossible to establish any 
exact connection between such results and the functional or growth effect 
that can be traced directly to heat. As a consequence, temperature sums 
do not at present contribute anything of value to an understanding of the 
relation between cause and effect. The mean daily temperature is readily 



TEMPERATURE 7 1 

obtained by averaging twenty-four hour-temperatures recorded by the ther- 
mograph. The method employed by Meyen 1 , of deriving the mean 
directly from the maximum and minimum for the day, is not accurate ; from 
a large number of computations, the error is always more than two degrees. 
On the other hand, the mean obtained by averaging the maximum and 
minimum for the day and night has been found to deviate less than I de- 
gree from the mean proper. This fact greatly increases the value of maxi- 
mum-minimum instruments if they are read daily at 6:00 a.m. and 6:00 p.m. 

103. Temperature curves. The kinds and combinations of temperature 
curves are almost without number. The simple curves of most interest 
are those for a series of stations or habitats, based upon the level of three 
feet, or the surface, or the daily mean. The curves for each station represent- 
ing the different heights and depths and the season curve of the daily means 
for a habitat are also of much importance. One of the most illuminating 
combinations is that which groups together the various level curves for a 
series of habitats. Other valuable combinations are obtained by grouping 
the curves of daily means of different habitats for the season, or the var- 
ious station curves. 

104. Plant temperatures. The direct effects of temperature as seen in 
nutrition and growth can be ascertained only by determining the tempera- 
ture of plant tissues. The temperatures of the air and of the soil surface 
have an important effect upon humidity, and water-content, and through 
them upon the plant, but heat can influence assimilation, for example, only in 
so far as it is absorbed by the assimilating tissue. The temperatures of 
the leaf, as the most active nutritive organ of the plant, are especially im- 
portant. While it is a well-known fact that internal temperatures follow 
those of the air and soil closely, though with varying rapidity of response, 
this holds less for leaves than for stems and roots. Owing to the very 
obvious difficulties, practically nothing has yet been done -in this important 
field. A few preliminary results have been obtained at Minnehaha, which 
serve to show the need for such readings. Gravel slide rosettes in an air 
temperature of 24 ° C. and a surface temperature of 40 C. gave the follow- 
ing surface readings: Parmelia, 40 , Eriogonum, 38.6 , Arctostaphylus, 
35 , Thlaspi, 31.8 , and Senecio, 31 °. The leaf of Eriogonum Havum, 
w r hich is smooth above and densely hairy below, indicated a temperature of 
31.8° when rolled closely about the thermometer bulb with the smooth sur- 
face out, and 28 ° when the hairy surface was outside. The surface read- 

J Meyen, F. J. F. Grundriss der Pflanzengeographie, 12. 1836. 



72 THE HABITAT 

ings of the same leaf were .5°-i° higher when made upon the upper smooth 
surface. This immediately suggests that the lower surface may be modified 
to protect the leaf from the great heat of the gravel, which often reaches 

50°' C. (122° R). 

PRECIPITATION 

105. General relations. As the factor which exerts the most important 
control upon water-content and humidity, rainfall must be carefully con- 
sidered by the ecologist. It is such an obvious factor, and is usually spoken 
of in such general terms that the need of following it accurately is not evi- 
dent at once. When it is recognized that the fluctuations of water-content 
are directly traceable to it, it becomes clear that its determination is as 
important as that of any indirect factor. This does not mean, however, 
that the amount of yearly rainfall is to be taken from the records of the 
nearest weather station, and the factor dismissed. Like other instruments, 
the rain gauge must be kept at the base station of the area under study, 
and when this is extensive or diverse, additional instruments should be 
put into commission. While the different parts of the same general cli- 
matic region may receive practically the same amount of precipitation dur- 
ing the year, it is not necessarily true that the rainfall of any particular 
storm is equally distributed, especially in the mountains. Nothing less 
than an exact knowledge of the amount of rain that falls in the different 
areas will make it possible to tell how much of the water-content found at 
any particular time in these represents merely the chance differences of 
precipitation. 

The forms of precipitation are rain, dew, hail, snow, and frost. Of 
these, hail is too infrequent to be taken into account, while frost usually 
occurs only at the extremes of the growing season, and in its effect is 
rather to be reckoned with temperature. Snow rarely falls except during 
the period of rest, and, while it plays an important part as cover, it is 
merely one of several factors that determine the water-content of the soil at 
the beginning of spring. The influence of dew is not clearly understood. It 
is almost always too slight in amount and too fleeting to affect the water- 
content of the soil. It seems probable that it may serve by its own evapora- 
tion to decrease in some degree the water loss from the soil, and from be- 
dewed plants. If, however, the dew is largely formed by the water of the 
?oil and of the plant, as is thought by some, then it is negligible as a re- 
inforcement of water-content. From the above, it is evident that rainfall 
alone exerts a profound effect upon the habitat, and it is with its measure- 
ment that the ecologist is chiefly concerned. 



PRECIPITATION 



73 



106. The rain gauge, as the illustration shows, is a cylindrical vessel with 
a funnel-shaped receiver at the top, which is 8 inches in diameter. The re- 
ceiver fits closely upon a narrower brass vessel or measuring tube in which 
the rain collects. The ratio of surface between receiver and tube is io to i. 
For readings covering a general area, the rain -gauge is placed in the open, 
away from buildings or other obstructions, and is sunken in the ground suffi- 
ciently to keep it upright. In localities where winds are strong, it is usually 
braced at the sides also or supported by a wooden frame. In measuring 
the amount of rain in the measuring tube, the depth is divided by ten in 



front Vieio. 



VerticaZ Sectunu 




Horizontal SectCarc^B-F 



Fig. 22. Rain gauge showing construction. 

order to ascertain the actual rainfall. The depth is measured by inserting 
the measuring-rod through the hole in the funnel until it touches the bot- 
tom. It is left for a second or so, quickly withdrawn, and the limit of the 
wetted portion noted. In the case of standard rods, the actual rainfall is 
read directly in hundredths, so that the division by ten is unnecessary. 
After each reading, the measuring-tube is carefully drained, replaced, and 
the receiver put in position. Xo regular time for making readings is neces- 
sary. During a rainy period, it is customary to make a measurement each 
day, but it has been found more satisfactory for ecological purposes to 
measure each shower, and to record its duration. These two facts furnish 



74 



THE HABITAT 



a ready clue to the relative amount of run-off in each fall of rain. The 
measurement of snowfall is often made merely by determining its depth. 
For comparison with rainfall, the rain gauge with receiver and tube with- 
drawn is used. The snow which falls is melted, poured into the measur- 
ing tube, and measured in the ordinary way. The U. S. Weather Bureau 
standard rain gauge, with measuring stick, may be obtained of H. J. Green, 
or of J. P. Friez for $5.25. 

107. Precipitation records. From the periodic character of precipitation, 
rainfall sums, means, and curves have little importance in the careful study 
of the habitat. The rainfall curve for the growing season is an aid in ex- 
plaining the curve of water-content, and the mean rainfall of a region gives 
some idea of its vegetation, though even here the matter of its distribution 
is of primary importance. The rain and snow charts published by the U. 
S. Weather Bureau furnish data of some importance for the general study 
of vegetation, but it is evident that they can play little part in a system 
which is founded upon the habitat. Precipitation records, for reasons of 
brevity and convenience, are united with wind records, and the form will 
be found under the discussion of this factor. 



WIND 

108. Value of read=> 
ings. On account of its 
direct effect upon hu- 
midity, and its conse- 
quent influence upon 
water-content, the part 
which wind plays in a 
habitat can not be 
ignored in a thorough 
investigation. It is an 
important element in ex- 
posure, and accordingly 
has a marked mechani- 
cal effect upon the 'vege- 
tation of exposed habi- 
tats, alpine slopes, sea- 
coasts, plains, etc. 
Owing to its inconstancy and its extreme variation in velocity, single wind 
readings are absolutely without value. When read in series, anemometers 
give some information upon the comparative air movement in different hab- 




Fig. 23. Simple anemometer. 



WIND 



75 



itats, but the chance of error is great, except when the breeze is steady. 
Anemographs alone give real satisfaction. Accurate results, however, are not 
obtainable without a series of two or more in different habitats, and it is 
still an open question whether the results obtained justify the expense. For 
a completely equipped base station, anemometer, anemograph, and wind 
vane are desirable instruments, but the study of the habitat has by no 
means reached the stage of precision in which their general use is 
necessary. 

109. The anemometer in its simplest form is adapted only to readings 
made under direct observation, as a sudden change in the direction of the 
wind reverses the move- 
ment of the indicator 
needle. This simple wind 
gauge, shown in figure 
23, has been used for in- 
structional purposes, and 
to a slight extent, also, in 
ascertaining the effect of 
cover. In constant winds, 
successive single readings 
are found to have value, 
but, ordinarily, the obser- 
vations must be simultane- 
ous. Careful tests of this 
simple instrument show that 
it is essentially accurate. 
It may be obtained from 
the C. H. Stoelting Com- 
pany, 31 W. Randolph St., 
Chicago, for $25. The 
standard anemometer 
(Fig. 24) is practically a 
recording instrument up to 1,000 miles, but as the dials run on without 
any indication of the total number of revolutions, it must be visited and 
read each day. This renders its use difficult for habitats which are some 
distance apart. When exact determinations of wind values become neces- 
sary, the most successful method is to establish a series of three standard 
anemometers. One of these should be placed upon the most exposed part 
of a typically open habitat, the second in the most protected part of the 
same habitat, while the third is located in the midst of a representative 
forest formation. If the two habitats are close together, the daily visits 




Fig. 24. Standard anemometer. 



7 6 



THE HABITAT 



can be made without serious inconvenience. The reading of the register* 
ing dials requires detailed explanation, and for this the reader is referred 
to the printed directions which accompany the instrument. In setting up 
the anemometer it must be borne in mind that the ecologist desires the wind 
velocity for a particular habitat. In consequence, the precautions which the 
meteorologist takes to place the instrument at a certain height and well 
away from surrounding obstructions do not hold here. Standard anemo- 
meters are furnished by H. J. Green, and J. P. Friez for $25 each. 

The anemograph is an anemometer electrically connected with an auto- 
matic register. It is the only instrument adapted to continuous weekly 
records in different habitats, but the price, $75 ($25 for the anemometer 
and $50 for the register) is practically prohibitive, at least until a complete 
series of ecographs for other factors has been obtained. 

110. Records. The following form is used as a combined record for 
precipitation and wind: 





1 




a 


►a 

< 


V 

U 

O 
P. 

X 
W 


H 


RAINFALL 


cd 

pq 


WIND 




Q 


cj 




6 




^ O 


V 

cd 


29/8/04 

31/8/04 

2/9/04 

3/9/04 


6:30 p.m. 
5:45 p.m 
4:00 p.m. 
10:00 p.m 


Half gravel 

u 


Hiawatha 
it 


2550 m. 


N.E. 17° 


Asterare 
it 


1 

Trace 

.2 
Trace 


8 hours 
10 ruin 
2 hours 


5 
12 

7 
18 


3 ft. 


N.W. 
W. 













SOIL 

111. Soil as a factor. In determining the value of the soil as a factor in 
a particular habitat, it must be clearly recognized that its importance lies 
solely in the control which it exerts upon water-content and nutrient-con- 
tent. The former is directly connected with the texture or fineness of the 
soil, the latter with its chemical nature. Accordingly, the structure of the 
soil and its chemical composition are the fundamental points of attack. 
These are not at all of equal value, however. Water is both a food, and a 
solvent for the nutrient salts of the soil. Furthermore, the per cent of sol- 
uble salts, as determined in mechanical analyses, is practically the same for 
all ordinary soils. Indeed, the variations for the same soil types are as 
great as for entirely different types. For these reasons, soluble salt-con- 
tent may be ignored except where it is readily seen to be excessive, as in 
alkaline soils ; and determinations of chemical composition are necessary 
only in those soils which contain salts or acids to an injurious degree, e. g., 



soil 77 

alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, 
on the other hand, in the usual absence of excessive amounts of solutes, 
absolutely controls the fate of the water that enters the ground, in addition 
to its influence upon the run-off. It determines the amount of gravitation 
water lost by percolation, as well as the water that can be raised by capil- 
larity. The resultant of these, the total soil water or holard, is hence an 
effect of structure, while the size and compactness of the particles are con- 
clusive factors in controlling* the chresard. It must be recognized, how- 
ever, that these are all factors which enable us to interpret the amount of 
holard or chresard found in a particular soil. They have no direct impor- 
tant effect upon the plant, but influence it only in so far as they affect the 
water present. 

112. The value of soil surveys. The full appreciation of the preeminent 
value of water-content, particularly of the chresard, greatly simplifies the 
ecological study of soils. The ecologist is primarily concerned with soil 
water only in its relation to the plant, and while a fair knowledge of soil 
structure is essential to a proper understanding of this, he has little concern 
with the detailed study of the problems of soil physics. For the sake of a 
proper balance of values, he must avoid the tendency noted elsewhere of 
ignoring the claims of the plant, and of studying the soil simply as the seat 
of certain physical phenomena. Accordingly, it is felt that mechanical and 
chemical analyses, determinations of soluble salt-content, etc., have much 
less value than has been commonly supposed. The usual methods of soil 
survey, which pay little or no attention to water-content, and none at all to 
available water, are practically valueless for ecological research. This state- 
ment does not indicate a failure to appreciate the importance of the usual 
soil methods for many agricultural problems, such as the use of fertilizers, 
conservation of moisture, etc., though even here to focus the work upon 
water-content would give much more fundamental and serviceable results. 
For these reasons, slight attention will be paid to methods of mechanical and 
chemical analysis. In their stead is given a brief statement of the origin, 
structure, and character of soils with especial reference to water-content. 

113. The origin of soils. Rocks form soils in consequence of weathering, 
under the influence of physical and biotic factors. Weathering consists of 
two processes, disintegration, by which the rock is broken into component 
particles of various sizes, and decomposition, in which the rock or its frag- 
ments are resolved into minute particles in consequence of the chemical 
disaggregation of its minerals, or of some other chemical change. These 
processes are usually concomitant, although, as a rule, one is more evident 
than the other. The relation between them is dependent upon the character 



7§ THE HABITAT 

of the rock and the forces which act upon it. Hard rocks, i. e., igneous and 
metamorphic ones, as a rule disintegrate more rapidly than they decom- 
pose ; sedimentary rocks, on the other hand, tend to decompose more rapidly 
than they disintegrate. In many cases the two processes go hand in hand. 
This difference is the basis for the distinction, first proposed by Thurmann, 
between those rocks which weather with difficulty and those which weather 
readily. The former were called dysgeogenous, the latter eugeogenous. 
Thurmann restricted the application of the first term to those rocks which 
produce little soil, but it seems more logical to apply dysgeogenous to 
those in which disintegration is markedly in excess of decomposition, and 
eugeogenous to those rocks that break down rather readily into fine soils. 
With respect to the general character of the soil formed, rocks are pelogen- 
ous, clay-producing, psarnmogenous, sand-forming, or pelopsammogenous, 
producing mixed clay and sand. The first two are divided into perpelic, 
hemipelic, oligopclic, perpsammic, etc., with reference to the readiness with 
which they are -weathered, but this distinction is not a very practicable 
one. The grouping of soils into silicious, calcareous, argillaceous, etc., with 
reference to the chemical nature of the original rock, is of no value to the 
ecologist, apart from the g'eneral clue to the physical properties which it 
furnishes. 

114. The structure of soils. The water capacity of a soil is a direct 
result of the fineness of the particles. Since the water is held as a thin 
surface film by each particle or group of them, it follows that the amount 
of water increases with the water-holding surface. The latter increases as 
the particles become finer and more numerous, and thus produce a greater 
aggregate surface. The upward and downward movements of water in the 
soil are likewise in immediate connection with the size of particles. The 
upward or capillary movement increases as the particles become finer, thus 
making the irregular capillary spaces between them smaller, and magnify- 
ing the pull exerted. On the contrary, the downward movement of gravita- 
tion water, i. e., percolation, is retarded by a decrease in the size of the soil 
grains and hastened by an increase. Hence, the two properties, capillarity 
and porosity, are direct expressions of the structure of the soil, i. e., of its 
texture or fineness. Capillarity, however, increases the water-content of the 
upper layers permeated by the roots of the plant, while porosity decreases 
it. On the basis of these properties alone, soils would fall into two groups, 
capillary soils and porous soils, the former fine-grained and of high water- 
content, the latter coarse-grained and with relatively little water. A third 
factor, however, of great importance must be taken into account. This is 
the pull exerted upon each water film by the soil particle itself. This pull ap- 



SOIL 



79 



parently increases in strength as the film grows thinner, and explains why it 
finally becomes impossible for the root-hairs to draw moisture from the soil. 
This property, like capillarity, is most pronounced in fine-grained soils, such 
as clays, and is least evident in the coarser sands and gravels. It seems to 
furnish the direct explanation of non -available water, and, in consequence, 
to indicate that the chresard is an immediate result of soil texture. 

115. Mechanical analysis. From the above it is evident that, with the 
same rainfall, coarse soils will be relatively dry, and fine soils correspondingly 
moist. However, this difference in holard is somewhat counterbalanced by 
the fact that the chresard is much greater in the former than in the latter. 
The basis of these relations can be obtained only from a study of § the tex- 
ture of the soil. The usual' method of doing this is by mechanical analysis. 
This is far from satisfactory, since the use of the sieves often brings about 
the disaggregation of groups of particles which act as units in the soil. 
Furthermore, the analysis affords no exact evidence of the compactness of 
the soil in nature, and tests of capillarity and porosity made with soil sam- 
ples out of position are open to serious error. Nevertheless, mechanical 
analyses furnish results of some value by making it possible to compare soils 
upon the basis of texture. For ecological purposes, mi- 
nute analyses are undesirable; their value in any work 
is doubtful. A separation of soil into gravel, sand, and 
silt-clay is sufficient, since the relative proportion of 
these will explain the holard and chresard of the soil 
concerned. The latter are also affected in rich soils, 
especially of forests, by the organic matter present. If 
this is in a finely divided condition, the amount is de- 
termined by calcining. When a definite layer of leaf- Fig. 25. Sieves for 
mold is present, as in forests and thickets, its water-value 

is found separately, since its power of retaining water is altogether out of 
proportion to its weight. 

116. Kinds of soils. It is very doubtful whether it is worth while to at- 
tempt to distinguish soils upon the basis of mechanical analysis. Un- 
questionably, the most satisfactory method is to distinguish them with 
respect to holard and chresard, and to regard texture as of secondary im- 
portance. A series of soil classes which comprise various soil types has 
been proposed by the U. S. Bureau 1 of Soils as follows : ( i ) stony loam, 
(2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, 
(7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) 
silt loam, (12) clay loam, (13) clay, (14) adobe. These are based 




instructions to Field Parties and Descriptions of Soil Types, 35. 1903. 



SO THE HABITAT 

entirely upon mechanical analyses, and in some cases are too closely related 
to be useful. The line between them can nowhere be sharply drawn. In- 
deed, the variation within one class is so great that soils have frequently 
been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per 
cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely re- 
lated to Oxnard sandy loam (26-37-1(8-12) and to Afton fine sandy loam 
(28-43-18-8) than to Coral sand (61-29-3-4), Galveston sand (6-91- 
1-1), or Salt Lake sand (84-1 5-1-0). Elsinore sandy loam (8-38- 
35-10) is much nearer to Hanford fine sandy loam (9-36-33-14) than to 
Billings sandy loam (1-60-22-11) or to Utuado sandy loam (48-23- 
19-8). The soil types are much more confused, and for ecological pur- 
poses at least are entirely valueless. Lake Charles fine sandy loam has 
the composition, 1-34-52-9 ; Vernon fine sandy loam, 1-37-54-7, while 
many other so-called types show nearly the same degree of identity. 

117. The chemical nature of soils. The effect of alkaline and acid sub- 
stances in the soil upon water-content and the activities of the plant is 
far from being well understood. It is generally recognized that salts and 
acids tend to inhibit the absorptive power of the root-hairs. In the case 
of saline soils, this inhibitive effect seems to be established, but the action 
of acids in bogs and swamps is still an open question. It is probable that 
the influence of organic acid has been overestimated, and that the curious 
anomaly of a structural xerophyte in a swamp is to be explained by the 
stability of the ancestral type and by the law of extremes. Apart from the 
effect which excessive amounts of acids and salts may have in reducing 
the chresard, the chemical character of the soil is powerless to produce 
structural modification in the plant. Since Thurmann's researches there 
has been no real support of the contention that the chemical properties of 
the soil, not its physical nature, are the decisive factors in the distribution 
and adaptation of plants. It is not sufficient that the vegetation of a 
silicious soil differs from that of a calcareous one. A soil can modify the 
plants upon it only though its water-content, or the solutes it contains. 
Hence, the chemical composition of the original rock is immaterial, except 
in so far as it modifies these two factors. Humus, moreover, while an 
important factor in growth, has no formative influence beyond that which 
it exerts through water-content. 

PHYSIOGRAPHY 

118. Factors. The physiographic factors of a definite habitat are altitude, 
exposure, slope, and surface. In addition, topography is a general though 
less tangible factor of regions, while the dynamic forces of weathering, 



PHYSIOGRAPHY 8 1 

erosion, and sedimentation play a fundamental role in the change of habi- 
tats. It is evident, however, that these, except where they affect the de- 
struction of vegetation directly, can operate upon the plant only through 
more direct factors, such as water, light, and temperature. While they are 
themselves not susceptible of measurement, they can often be expressed 
in terms of determinable factors, i. e., slope, exposure, and surface. Fun- 
damentally, they constitute the forces which change one habitat into another, 
and, in consequence, are really to be considered as the factors which pro- 
duce succession. The static features of physiography, altitude, etc., lend 
themselves readily to determination by means of precise instruments. These 
factors, though by no means negligible, are remote, and consequently their 
mere measurement is insufficient to indicate the nature or extent of their 
influence upon the plant. It is necessary to determine also the manner 
and degree in which they affect other factors, a task yet to be done. 
Readings of altitude, slope, and exposure are so easily made that the stu- 
dent must carefully avoid the tendency to let them stand at their own 
value, which is slight. Instead, they should be made the starting point for 
ascertaining the differences which they produce in water-content, humidity, 
wind, and temperature. 

Altitude 

119. Analysis into factors. Of all physiographic features, "altitude is the 
most difficult to resolve into simple factors. Because of general geographic 
relations, it has a certain connection with rainfall, but this is vague and 
inconstant. Obviously, in its influence upon the plant, altitude is really 
pressure, and in consequence its effect is exerted upon the climatic and 
not the edaphic factors of the habitat. Theoretically, the decrease of air- 
pressure in the increased altitude directly affects humidity, light, and tem- 
perature. Actually, while there is unquestionably a decrease in the ab- 
sorption of the light and heat rays owing to the fact that they traverse 
less atmosphere, which is at the same time less dense, this seems to be 
negligible. Photometric readings at elevations of 6,000 and 14,000 feet 
have so far failed to show more than slight differences, which are alto- 
gether too small to be efficient. The effect upon humidity is greater, but the 
degree is uncertain. Continuous psychrographic records at different ele- 
vations for a full season, at least, will be necessary to determine this, since 
the psychrometric readings so far made, while referred to a base psychro- 
graph, are too scattered to be conclusive. Finally, the length of the sea- 
son, itself a composite, is directly dependent upon the altitude. This rela- 
tion, though obscure, rests chiefly upon the rarefaction of the air which 
prevents the accumulation of heat in both the soil and the air. 



82 



THE HABITAT 



120. The barometer. To secure convenience and accuracy in the de- 
termination of altitude, it is necessary to use both a mercurial and an 
aneroid barometer. The latter is by far the most serviceable for field work, 
but it requires frequent standardizing by means of the former. The mer- 
curial form is much more accurate and should be read daily in the base 
station. It is practically impossible to carry it in the field, except in the 
so-called mountain form, which is of great service in establishing the alti- 
tudes of a series of stations. In use the aneroid barometer may be checked 
daily by the mercurial standard, or it may be set at the altitude of the base 
station, thus giving a direct reading. After the normal pressure at the 
base has once been ascertained, however, the most satisfactory method is 
to set the aneroid each day by the standard, at the same time noting the 

pressure deviation in feet of elevation (see 
p. 46). The absolute elevation of the var- 
ious stations of a series may be determined 
either by adding or subtracting this devia- 
tion from the actual reading at the station, 
or by noting the change from the base 
station, and then adding or subtracting this 
from the normal of the latter. When it is 
impossible to check the aneroid by means 
of a mercurial barometer, the average of a 
series of readings made at different days at 
one station, especially if taken during 
settled weather, will practically eliminate 
the daily fluctuations, and yield a result 
essentially accurate. Even in this event, 
the accuracy of the aneroid should be 
checked as often as possible, since the 
mechanism may go wrong at any time. The 
barograph, while a valuable instrument for base stations, is not at all neces- 
sary. These instruments can be obtained from all makers of meteorological 
apparatus, such as H. J. Green, and J. P. Friez. Aneroid barometers reading 
to 16,000 feet cost about $20; the price of the Richards aneroid barograph is 
$45. Ordinary observatory- barometers cost $30-340; the standard instru- 
ment sells at $75-$ 1 00. The mountain barometer, which is altogether the 
most serviceable for the ecologist, ranges from $30-$55, depending upon 
accessories, etc. 




Fig. 26. Aneroid barometer. 



PHYSIOGRAPHY 



83 



Slope 

121. Concept. This term is used in the ordinary sense to indicate the 
relation of the surface of a habitat to the horizon. Although it is a com- 
plex of factors, or rather influences several factors, these are readily de- 
terminable. The primary effect of 
slope is seen in the control of run- 
off and drainage, and consequently 
of water-content, although these 
are likewise affected by soil tex- 
ture and by surface. Slope, more- 
over, as a concomitant of exposure, 
has an important bearing upon 
light and heat by virtue of deter- 
mining the angle of incidence, and 
also upon wind, and, through it, 
upon the distribution of snow. At 
present, while it can be expressed 
definitely in degrees, it has not yet 
been connected quantitatively with 
more direct factors. This is, how- 
ever, not a difficult 'task, and it is 
probable that we shall soon come 
to express slope principally in 
amount of run-off, and of incident 
heat. 



122. The clinometer. In the 

simplest form, this instrument is 
merely a semicircle of paper, with 
each half graduated from 1-90 °. It 
is mounted on a board and placed 
base upward, upon a wooden strip, 
2 feet long and 2 inches wide, 
which has a true edge. At the cen- 
ter of the circle is attached a line 
and plummet for reading the per- 
pendicular. A more convenient 
form is shown in figure 28, which 
is both clinometer and compass. 
This also necessitates the use of a 




Fig. 27. Mountain barometer: (a) in carry- 
ing case; (b) set up for use. 



8 4 



THE HABITAT 



basing strip to eliminate the inequalities of the surface. The dial face 
is graduated to show inches of rise per yard, as 'well as the number of de- 
grees, but the latter, as the simpler term, is preferable for ecological work. 
In making a reading, the basing strip is placed upon a representative area 
of the slope, and pressed down firmly to equalize slight irregularities. The 
clinometer is moved slightly along the upper edge, causing the marker to 
swing freely. After the latter comes to rest, the instrument is carefully 
turned upon its back, when the angle of the slope in degrees may be read 
directly. Two or three such readings in different areas will suffice for the 
entire habitat, unless it be extremely irregular. The clinometer with com- 
pass may be obtained from the Keuffel and Esser Company, in Madison 
St., Chicago, Illinois, for $5. 





Fig. 28. Combined clinometer and compass. 



123. The trechometer. For measuring the effect of slope upon run-off, 
a simple instrument called the trechometer (rp^x®, to run off) has been 
devised. This consists merely of a metal tank, 3 x 4 x 12 inches, hold- 
ing 144 square inches of water, with an opening ^ x 12 inches at the base 
in front, closed by a tight-fitting slide. Three metal strips, 2 x 12 inches, 
are fastened to the front of the tank in such a way as to enclose a square 
foot of soil into which the strips penetrate an inch. In the front strip is an 
opening, 1 inch square, provided with a drip from which the run-off is 
collected in a measuring vessel. In use, the instrument is put in position 
with the metal rim forced down 1 inch into the soil ; the tank is filled, the 
graduate put in place, and the slide raised. The run-off for a square foot 
is the amount of water caught by the graduate, and is represented in 
cubic inches per square foot. F'or obtaining results which express slope 
alone, comparisons must be made upon the same soil, from which all cover, 
dead and living, has been removed. They must be as closely together in 
time as possible, at least during the same day, as rain or evaporation will 



PHYSIOGRAPHY 85 

cause considerable error. It is obvious that with the same slope or on a 
level the trechometer may also be used to advantage to determine the ab- 
sorptive power of soils of different texture. It serves well a similar pur- 
pose when used in different habitats to measure the composite action of 
slope, soil, and cover in dividing the rainfall into run-off and absorbed 
water. 

Exposure 

124. Exposure refers primarily to the direction toward which a slope 
faces, i. e., its exposition or insolation with respect to sun and wind. It is 
not altogether separable from slope, however, inasmuch as the angle of the 
slope has some effect upon the degree of exposure. The chief influence of 
exposure is exerted through temperature, since slopes longest exposed to 
the sun's rays receive the most heat. This is supplemented in an important 
deg'ree by the fact that a group of rays i foot square will occupy this area 
only on slopes upon which they fall at right angles. In all other cases the 
rays are spread over a longer area, with a consequent reduction in the 
amount of heat received. This effect is felt principally in evaporation from 
the soil, and in soil temperatures. For the leaf, it is largely if not entirely 
negligible, since the angle of incidence is determined by the position of 
the leaf, which is the same for each species whether on the level or upon 
a slope. On this account, exposure has little or no bearing upon light, 
except that the total amount of light received by the aggregate vegetation 
of a slope will be greater than for a level area of the same size. The 
effect of wind varies with the exposure. It is naturally most pronounced^ 
in those directions from which the prevailing dry or cold winds blow, and 
it is greatly emphasized by the fact that the opposite exposure is corres- 
pondingly protected. The influence of wind, especially in producing 
evaporation from the plant and the soil, increases with the slope, since 
the mutual protection of the plants, or that afforded the soil by the cover, 
is much reduced. Finally, the distribution of the snow by the wind, a 
matter of considerable importance for early spring vegetation, is largely 
determined by exposure. 

Exposure is expressed directly in terms of direction, to which is added 
the angle of the slope. A good field compass, reading to twelve points, is 
sufficient. It should be checked, of course, by the declination of the needle 
at the place under observation. A convenient instrument is the one already 
mentioned, in which compass and clinometer are combined, since these are 
regularly used at the same time. 

125. Surface. The most important consideration with respect to surface 
is the presence or absence of cover, and the character of the latter. With 



86 



THE HABITAT 



the exception of snow, cover is, however, a question of vegetation, living 
and dead, and consequently is to be referred to the discussion of biotic 
factors. The surface of the soil itself often shows irregularities which 
must be taken into account. Such are the rocks of boulder and rock fields, 
the hummocks of meadows and bogs, the mounds of prairie dog towns, 
the innumerable minute gullies and ridges of bad lands, the raised tufts 
of sand-hills, etc. The influence of these is not profound, but they do 
have an appreciable effect upon the run-off, temperature, and wind. In 
many cases, this is distinctly measurable, but as a rule little more can be 
done than to indicate that the surface is even or uneven, and to describe 
the degree and kind of unevenness. 

126. Record of physiographic factors. Altitude, slope, exposure, and 
surface are essentially constant factors, and are determined once for all, 
after a few check readings have been made, except in those relatively rare 
habitats in which dynamic forces are very active. The form of record 
used is the following : 



DATE 


FORMATION 


STATION 


GROUP 


ALTITUDE 


SLOPE 


EXPOSURE 


SURFACE 


10/7/02 


Gravel slide . . 


Golf L,inks. .. 


Eriogouare. .. 


2700 m. 


23o 


N.N.W. 


Even 


" 


Brook bank.. 


Jack Brook. .. 




2550 m. 


50 


E.N.E. 


" 


" 


Half gravel . . 


Hiawatha 


Achilleare 


2600 m. 


LP 


E. 


Uneven 


n 




Milky Way... 


Opulasterare. 


2625 m. 


12° 


N. 


Even 



127. Topography. As heretofore indicated, questions pertaining -to the 
form and development of the land concern groups of habitats within which 
each habitat is the unit of investigation after the manner already laid down. 
A knowledge of topography is essential to the accurate mapping of a 
region, for which the simple methods of plane table and contour work are 
employed, while the geology of the surface is of primary importance in 
the study of successions. 

BIOTIC FACTORS 



128. Influence and importance. Biotic factors are animals and plants. 
With respect to influence they are usually remote, rarely direct. Neverthe- 
less, they often play a decisive part in the vegetation. Their effect is, as 
a rule, felt directly by the formation rather than the habitat, but in either 
case the one reacts upon the other. Such factors are not themselves sus- 
ceptible of exact measurement, but their influence upon the habitat is 
usually measurable in terms of the physical factors affected. In the case 



BIOTIC FACTORS 87 

of biotic factors, it must be distinctly understood that these are not properly 
factors of the habitat as a physical complex, but that they are rather to be 
considered as reactions exerted by the effect, or formation, upon the cause 
or habitat. This is most especially true of plants. 

129. Animals. The activities of man fall into two classes: (i) those 
that destroy vegetation, and (2) those that modify it. There are rare in- 
stances also where the work of man has changed a new or already denuded 

• habitat. In the cases where the vegetacion is destroyed, the habitat itself 
is sufficiently changed to permit the effect to be measured by physical factor 
instruments. Otherwise, the influence is felt only by the formation, as when 
man makes possible the migration of weeds, and it can be measured in 
terms of invasion by the quadrat alone. It becomes especially evident, 
then, in the case of man's activities, that where they produce a denuded « 
habitat they are to be regarded as factors in the habitat ; when they merely 
affect the formation, this is not strictly true. The changes wrought by 
other animals are essentially the same as those produced by man. They 
are not so marked nor so important, but their relation to habitat and forma- 
tion is the same. As a rule, however, they affect the habitat much less 
than they do the formation. 

130. Plants. As a dead cover, vegetation is a factor of the habitat 
proper, but it has relatively little importance, since it occurs regularly dur- 
ing the resting period. Its chief effects are in modifying soil temperature, 
and in holding snow and rain, and thereby increasing the water-content. 
By its gradual decay, moreover, it not only adds humus to the soil, but it 
thereby increases the water-retaining capacity of the latter also. The 
cover of living vegetation reacts upon the habitat in a much more vital 
fashion, exerting a powerful effect upon every physical factor of the 
habitat. The factors thus affected are distinctly measurable though it is 
often impossible to determine just how much of the factor is directly trace- 
able to the vegetation. This is a simple problem in the case of most aerial 
factors, especially light, but it is extremely difficult for soil factors, such 
as water-content and soil texture. In the case of all habitats covered with 
formations, by far the great majority, it is impossible as well as unneces- 
sary to separate the physical factors of the habitat proper from the re- 
action upon them which the plant covering exerts. Indeed, the great differ- 
entiation of habitats is largely due to the universal principle that in 
vegetation the effect or formation always reacts upon the cause or habitat in 
such a way as to modify it. As fundamental causes of succession, the discus- 
sion of the various reactions of vegetation is reserved for another place. 



88 



THE HABITAT 



Methods of Habitat Investigation 

131. The use of the various instruments previously described depends 
largely upon the preponderance of simple instruments or recording ones. 
The former necessitate a number of well-trained assistants; the latter re- 
quire only a part of the time of one investigator. For the most satis- 
factory results, however, an assistant is all but indispensable. Since sim- 
ple instruments are most easily obtained because of their cheapness, and 
are especially adapted to purposes of instruction, the method of using them 
will be described first, and then that of ecograph batteries. 

THE METHOD OF SIMPLE INSTRUMENTS 



132. Choice of stations. This method is based upon simultaneous read- 
ings by means of simple instruments in a series of habitats, or of stations 

l 




Fig. 29. Series of stations: I, at Minnehaha; II, at Lincoln in the prairie formation. 

in a single habitat. Such readings are necessary for -the variable atmos- 
pheric factors, humidity, light, temperature, and wind. Frequent read- 
ings suffice for water-content and precipitation, while only two or three 
determinations, enough to check out the error, are necessary for the con- 
stant factors, altitude, slope, exposure, and surface. An account of the 
exact procedure employed in class study at Lincoln and Minnehaha will 
best serve to illustrate the use of this method. The series of stations chosen 
at Lincoln were primarily within a single formation, for the purpose of 
determining the physical factor variation in different areas. One series 
was located in the prairie-grass formation (Koelera-Andropogon-psilium), 
and consisted of the following stations: (i) low prairie, (2) crest of ridge I, 



METHOD OF SIMPLE INSTRUMENTS 89 

(3) northeast slope of ridge I, (4) grassy -ravine, (5) southwest slope of 
ridge II, (6) bare crest of ridge II, (7) thicket ravine. The other series 
was established in the bur-oak-hickory forest (Quercus-Hicoria-hylium) 
at the following stations: (1) thicket, (2) woodland, (3) knoll in forest, 

(4) depression in forest, (5) level forest floor, (6) nettle thicket, (7) brook 
bank. At Minnehaha the series was primarily one of different formations : 
(1) the pine formation (Pinus •xcrohylhtm) , (2) the gravel-slide formation 
(Pseiidocymopterus-Mentselia-chalicium), (3) east slope of spruce forest 
(Picea-Psendotsuga-hylium), (4) ridge in the spruce forest, (5) north 
slope of spruce forest, (6) brook bank in forest, (7) the thicket formation 
(Quercus-Cercocarpus-lochmodium), (8) the aspen formation (Populus 
hylium). When permanent or temporary quadrats are established, they are 
ordinarily used as regular stations, since this enables one to refer the 
physical factor readings to a few definite individual plants, as well as to the 
entire formation. The transects in figure 29 illustrate two of the above 
series of stations. 

133. Time of readings. The frequency of simple readings and the 
times at which they are made must be regulated largely by opportunity and 
convenience. In addition to making readings once or twice a week through- 
out the season, the series should be read at least once every day for a rep- 
resentative week or two. It is also very desirable to have a series for each 
hour of a typical day, or of two days, one of which is clear, the other cloudy. 
When a single daily reading is made, it should be taken at or as near me- 
ridian as possible. The usual series is the one obtained by simultaneous 
observations at the same level in different stations. An important series is 
also secured by simultaneous readings at the various levels of the same 
station, though it is not necessary to take this series frequently. 

134. Details of the method. After the stations have been selected by a 
careful preliminary survey of the habitat or series of habitats, their location 
is indicated by a small flag bearing a number, in case there is no danger of 
these being disturbed. Otherwise, less conspicuous stakes are used. The 
ordinary practice is to visit each station of the series, and to take readings 
of water-content, altitude, slope, and exposure. On the first trip these are 
all made by the instructor, but after a short time the determination of each 
factor may be assigned in rotation to each of the students. After these 
constant factors have been read and recorded, one student is equipped with 
photometer, thermometer, and psychrometer, and, if desirable, anemometer, 
and left at the first station. At each succeeding station the same plan is 
followed, so that at the end of the series the constant factors have all been 



9Q 



THE HABITAT 



read, and there is an observer at each station prepared to make readings of 
the variable ones. The task of acquainting the students with the operation 
of photometer, psychrometer, etc., can best be done in class or at a previous 
field period, as it is evident that they must be familiar with the instruments 
before they can use them accurately in the field series. The details of oper- 
ation have already been given and need not be repeated here. The task of 
obtaining readings at the same moment may be met by supplying each 
observer with a watch, which runs exactly with all the others, or by making 
observations upon signal. The second means has been found most success- 
ful in practice, since the signal fixes the attention at the exact moment. 




Fig. 30. A denuded station in the aspen formation. 

The best plan is for the instructor to occupy a commanding position some- 
where near the middle of the series, and to give the signals by shout or 
whistle at the proper interval. Considerable care and experience are neces- 
sary to do the last satisfactorily. Sufficient time must be given for the 
"operation of the instrument and the making of the record. In addition, a 
period must be permitted to elapse which is long enough for every instru- 
ment to reach the proper reading. For example, in a series which contains 
a gravel slide and a forest, the thermometer which has just been used for 
an air reading will require four or five times as long an interval to respond 
to the temperature of the gravel as to that of the cool forest floor. In such 
series, the instructor should regularly take his place in the station where 



METHOD OF SIMPLE INSTRUMENTS 91 

the response is slowest or greatest. He must record the exact time of each 
signal, and note any general changes of sky or wind that produce temporary 
fluctuations at the time of reading. When the readings extend over a whole 
day, the usual plan is to begin at the last station and take a second series of 
water-content samples, noting the exact time in order that the rate of water 
loss may be determined. A check series of physiographic factors may be 
made at this time also, or this may be left for future visits. While it is un- 
necessary to take soil samples oftener than once a day, it is important to 
make at least one series at each visit. Sometimes it becomes desirable to 
know the rate of water loss in different stations during the day, and in this 
event, samples are taken at one or two hour intervals for the entire day. 

In making simultaneous readings at the different levels of one station, the 
observers are grouped in one spot in such a way that they do not interfere 
with the correct reading of each instrument. Readings of this sort are most 
valuable in the case of temperature, which shows greater differences at the 
various levels. Important differences of humidity and wind also are readily 
obtained, and, in layered formations, marked variations in the amount of 
light. In the open, the ordinary levels for temperature are 6 feet, 3 feet r 
surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6 
feet, 3 feet, and surface. In forests the same levels are used for comparison 
with formations in the open, but a more desirable series for light especially 
is secured by making readings at the height of, or better, just below the var- 
ious layers. Series of this sort are likewise made on signal. The best time of 
day is that of a period in which the middle station is read near meridian, 
since the variation due to time is sufficiently small to permit fairly accurate 
comparisons between the readings for the different stations. 

135. Records. The form used for recording the observations made by 

means of simple instruments is shown below. It is hardly necessary to state 
that it may be readily modified to suit the needs of different investigators. 
Ordinarily, each sheet is used for the records of one habitat or series alone, 
but for convenience sake, the records of two different series are here com- 
bined. The figures given are taken from records for the prairie and forest 
formations at Lincoln. 






92 



THE HABITAT 



Koelera-A n drop ogon-psilium 
April 25, 1901. Clear. South wind. 





TEMPERATURE 


LIGHT 


HUMIDITY 


WATER- 
CONTENT 


WIND 


TIME P.M. 


3:20 


3:24 


3:30 


3:35 


3:40 






3:45 


3:55 


% 


2:40 


3:05 










STATION 


l#m. 


Surf. 


5 


10 


15 


l^m. 


Surf. 


l&m. 


Surf. 


5 


10 


15 


l^m. 


Surf. 


1 


27.8 
26.5 
26.9 
26.2 

28 
28 
26.4 


29.6 
31.3 

28.5 

30 

32.4 

40.8 

30.8 


17.8 
18.3 
18.2 
16.8 
18.6 
23 . 8 
16 


15.8 

16.6 

14.2 

13.2 

14.6 

16 

13 


10.9 

12.8 

13.5 

11.6 

14.2 

15 

11 






57 
59 
58 
63 
59 
51 
68 


59 
59 
59 
66 
60 
51 
70 


17.7 
17.9 
16.5 
24.4 
10.7 
5 
27 


14.6 
12.2 
16! 9 
21.3 
17 

8.3 
24.3 


17.4 

14.3 

19 

24.8 

17.2 

10.3 

21.4 


740 
1100 

980 

920 
1080 
1010 

680 


280 


2 






510 


3 






520 


4 






460 


5 






490 


6 






410 


7 






52 











Quercus-Hicoria-hylium 
April 20, 1901. Clear. Southeast wind. 





TEMPERATURE 


LIGHT 


HUMIDITY 


WATER- 
CONTENT 


WIND 


TIME 
A.M. 


10:40 


10:46 


10:50 


10:55 


11:00 


12:00 


12:05 


11:10 


11:20 


i 


11:30 


11:45 


STA- 
TION 


iy 2 m. 


Surf. 


5 


10 


15 


l^m. 


Surf. 


l&m. 


Surf. 


5 


10 


15 


iy 2 m. 


Surf. 


1 


16 


25 


9.6 


8.4 


7.8 


.08 


.06 


73 


81 


24.2 


19.2 


19.5 


298 





2 


16.2 


30.5 


8.5 


8.4 


7.8 




11 


.09 


73 


86 


22 


22.5 


19.4 


375 


2 


3 


16.2 


17.8 


7.6 


7.8 


8 




08 


.06 


73 


95 


22.1 


20.4 


21.6 


640 


6 


4 


15.6 


26.2 


10.6 


8.4 


8.2 




06 


.03 


81 


95 


25.4 


23.1 


22.4 


275 


12 


5 


17.6 


25.4 


7.6 


7.4 


7.2 




03 


.02 


90 


95 


27.2 


19.8 


18.8 


178 


2 


6 


16.2 


20.2 


8.4 


7 


6.2 




02 


.01 


82 


90 


27.6 


20.8 


18.8 


115 


4 


7 


15.8 


17.2 


6.4 


6.4 


6.1 




05 


.04 


82 


90 


23.8 


19 


19.3 


60 






METHOD OF ECOGRAPH BATTERIES 

136. A battery of recording instruments consists of a selagraph, a psy- 
chrograph, and a thermograph, to which an anemograph is added when pos- 
sible. As stated before, the determination of water-content by the geotome 
method is more satisfactory than by any automatic instrument yet devised. 
When the base station is located where the sunlight is unobstructed, which 
should be the case whenever possible, it is unnecessary to include a selagraph 
in those batteries placed in similarly exposed stations, since the light values 
will be the same. As a rule, batteries are established within different zones 
or different habitats, except where a highly diversified habitat is made the 
subject of special inquiry. Such a restriction arises from the fact that ex- 
pense, care of operation, etc., place a limit upon the number of batteries, 



METHOD OF ECOGRAPH BATTERIES 93 

and, in such case, the task of primary importance is to establish the physical 
character of representative habitats. For these reasons, the first series of 
thermographs established in 1903 was located with respect to altitude, the 
instruments being- placed at Manitou 2,000 m., Minnehaha, 2,600 m., and 
Mount Garfield 3,800 m. In 1904. the stations established for the record of 
temperature and humidity were situated with respect to habitats represent- 
ing the four formations : gravel slide, half gravel slide, spruce forest, and 
brook bank. 

The batteries are located and set up according to the directions already 
given. A 2-meter quadrat with the battery as the middle is staked and 
mapped. Within this, all readings of water-content, soil temperature, and 
physiographic factors are made. Altitude, slope, exposure, and cover are 
recorded when each battery is located, and a soil sample is taken for me- 
chanical analysis. When the position of the batteries permits it, w T ater-con- 
ient readings should be made frequently, once or twice a week at least. In 
addition, a complete series of samples should be taken daily for a period 
sufficient to indicate the ordinary extremes of water-content. 

The ecograph battery of each habitat constitutes a standard to which the 
results obtained by simple instruments may be referred with accuracy. It 
not only does this, but it also serves as a basis for interpreting the readings 
of simple instruments in distant habitats of the same character. In this way 
a few batteries judiciously placed make possible the exact physical investiga- 
tion of a large number of habitats, covering a considerable area. The only 
limit, indeed, upon this method is that placed by time. The proportionate 
use of batteries and of simple instruments must be largely determined by 
the conditions which confront the investigator. It is obvious that, where 
expense is not a decisive factor, the gain in time and in completeness of 
results is enormously in favor of the battery. There is an additional value 
in the automatic and continuous record which can not be overlooked. When 
the use of instruments in the study of habitat and formation becomes uni- 
versal, the importance of the ecograph will be immeasurably enhanced. It 
will be possible to secure duplicate records of batteries located in the most 
remote and diverse regions, from the equator to the poles, and comparative 
phytogeography upon a scientific basis will for the first time be possible. 
This opens an alluring vista of the future when ecologists the world over 
will cooperate in such a way that the results obtained by ecograph batteries 
anywhere on the globe will permit of exact comparison. 



94 THE HABITAT 

THE EXPRESSION OF PHYSICAL FACTOR RESULTS 

137. The form of results. It is almost inevitable that the general adop- 
tion of precise methods of measuring the habitat will result in a common 
form for expressing the physical character of the latter. An actual diag- 
nosis of each habitat is not a difficult matter, after the factors are carefully 
measured, and will unquestionably lead to very desirable definiteness and 
precision. The accurate investigation of the physical factors of a number 
of habitats for one growing season furnishes the necessary material for a 
diagnosis based upon the mean for the growing season. Similar results for 
two or three seasons will yield a diagnosis as accurate and as final as that 
of a formation, or, indeed, as that of many species. The author's investi- 
gations have not yet gone far enough to warrant proposing a final form for 
this, but the following diagnosis is offered as a suggestion : 

Elymus-Miihleiibergia-chalicium. Habitat: holard 9 per cent, chresard 8 
per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel (gravel 
70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65 , surface 
82 , soil 59 , wind 10 miles, rainfall 8 inches, altitude 2,800 m., slope 23 °, 
exposure south, surface even, cover open, no active biotic agencies. 

The detailed comparison of habitats is made most readily by the graphic 
method of curves, which constitute the most desirable form of expression in 
connection with the original record upon which they are based. Factor 
means are particularly desirable for diagnostic purposes, and they furnish 
valuable curves also. Factor sums are impracticable at present, and it seems 
doubtful that they will ever be of much value. It is by no means impos- 
sible, however, that a more detailed and exact knowledge of the physiology 
of adaptation, coupled with methods of precision in the habitat, will render 
them necessary. 

Factor Records 

138. Experience has shown that the practice of making hasty and often 
formless records in the field is unwise and is apt to be inaccurate as well. The 
time saved in the field is more than counterbalanced by that consumed in 
copying the results into the permanent form. The danger of error in field 
notes rapidly taken is very grave, and the chance of confusion and the waste 
of time in deciphering them are great. Moreover, the task of checking a 
copy with the original, which is absolutely necessary for accuracy, involves 
a further expenditure of time and energy. For these reasons the field record 
should be made in permanent form. Definite record sheets are used, and 
the invariable rule is made that all readings are to be noted in ink at the 
time and spot where they are taken. On a long journey, or in the face of 



PHYSICAL FACTOR RESULTS 95 

many observations, the tendency to take notes or to record observations 
rapidly is very great, but this will correct itself after a few attempts to 
use such notes. The record forms for various factors have been indicated 
in the proper place, as well as the one for simultaneous readings. Ecograph 
sheets are carefully filed, and constitute permanent records. With a little 
practice they may be read almost as easily as tables, and any attempt to 
put them into tabular form is a mere Avaste of time. For purposes of study 
and of publication, it often becomes necessary to bring together all the 
results obtained for a particular habitat, both by simple instruments and by 
ecographs. The form of record used for this is essentially that already 
indicated for simultaneous readings on page 92, since general features and 
constant factors can not well be included in the table. Record sheets of 
this type have been printed at a cost of $5 per thousand, and the various 
factor records can be obtained at about the same rate. The size of sheet 
used is 9^ x 7^ inches. The record book is the usual note-book cover, which 
has been found neither too large nor too small. It is protected from dirt 
and rain by a covering of oilcloth which overlaps the edges. Record books 
should be carefully labeled, and each one should contain a single year's 
records. 

Factor Curves 

139. Plotting. The paper employed is divided into centimeter squares 
which are subdivided into 2-millimeter units. For ordinary curves the 
size of sheet is gy 2 x 754 inches, which makes it possible for curve sheets to 
be filed in the record book. Tablets containing 60 of these sheets can be 
obtained for 20 cents each from the Central School Supply House, Chicago. 
For curves longer than 9 inches special sizes of sheets must be used. On 
account of their inconvenience large sheets are avoided whenever possible. 
This can usually be accomplished by increasing the numerical value of the 
intervals. The inks employed in plotting are the waterproof inks of Chas. 
Higgins & Co., Brooklyn, New York. These are made in ten or more 
colors, black, violet, indigo, blue, green, yellow, orange, brown, brick red, 
carmine, and scarlet, and cost 25 cents per bottle. In addition to being 
waterproof, they make it possible to combine curves in all conceivable 
ways without destroying their identity. Furthermore, it is a great advan- 
tage to use the same color invariably for the same kind of curve : thus, 
it has been the practice to indicate the 3-foot, surface, 5, 10, and 15-inch 
temperature curves by violet, green, yellow, blue, and carmine respectively. 
A fine-pointed pen, such as the Spencerian No. 1, is most satisfactory for 
inking; drawing pens, such as Gillott's Crowquill, are too finely pointed for 
ordinary use. 



.96 THE HABITAT 

In plotting a curve, it is first necessary to determine the value of the 
interval, and the extreme range of the curve or combination. For example, 
in the case of temperature, it is most convenient to assign a value of i° 
Centigrade to each centimeter, since the thermometers used read to one- 
fifth of a degree, which corresponds exactly to the 2-millimeter units of each 
square. The length of the sheet permits a range of 22 degrees Centigrade, 
and the actual limits must be determined for the particular results to be 
employed. For the same region, it is very desirable that the unit interval 
and the range be the same, in order that all curve sheets may admit of direct 
comparison. Indeed, it is greatly to be hoped that in the future ecologists 
will agree to a uniform system of curve-plotting, cartography, etc., as 
the geographers are beginning to do in the construction of maps. The 
major intervals are written, or, better, typewritten, at both sides of the 
sheet, and the time or space intervals are indicated at the top. Each curve 
sheet is properly labeled, and essential data indicated. The readings are 
taken from the field record, and their proper positions indicated by a dot. 
These are connected first by a pencil line, the curves being made abrupt 
rather than flowing; and the line, after having been carefully checked, is 
traced in ink. 

140. Kinds of curves. Curves are named both with reference to the 
factor concerned and the position or sequence of the readings. The 
factors which lend themselves most readily to this method of representation 
are the variable ones, water-content, humidity, light, temperature, and 
wind, and corresponding curves are distinguished. Altitude and slope may 
likewise be shown by means of curves, but the use of cross section or con- 
tour lines serves the same purpose and is more natural. With regard to 
time and position, curves are distinguished as level, station, and point 
curves. A level curve is one based upon readings made at the same level 
through a series of stations or of habitats, e. g., the level curve of surface 
temperature. The station curve represents the various levels or points at 
which readings are made in a single station. The point curve has for a 
basis the hourly or daily variation of a factor at a particular point or level 
in a station. All of these may be simple curves, when established upon a 
single reading for a series, or mean curves when they are based upon the 
mean of a number of readings. Curves which show the extremes of a 
factor, i. e., the maximum and minimum, are also extremely valuable, 
though a combination of the two for comparison is preferable. 

141. Combinations of curves are invaluable for bringing similar curves 
together, and permitting ready comparison of them. For this, and also 
because they save space, they are regularly employed to the almost complete 



PHYSICAL FACTOR RESULTS 97 

exclusion of single curves. Combinations are made simply by tracing, the 
curves to be compared upon the same sheet, it being understood that dis- 
similar curves, e. g\, level and station, can not be combined. Colored inks 
are an absolute necessity in combining; the primary principle underlying 
their use is that curves-that approach closely or cross- each other must be 
traced in inks that contrast sharply. As elsewhere stated, it has been made 
the invariable rule to use the same color for the same level or point. This 
applies especially to temperature, but holds also for humidity, light, wind, 
and water-content, so that the color always indicates the level. For the 
same reason, it is applied to a combination of point curves for one station, 
though it is inapplicable to a series of point curves when these lie in the 
same level. Light readings above 6 feet and water-content readings below 
15 inches necessitate the use of additional colors. 

Combinations may be made of the curves of a single factor for purposes 
of comparison, or they may consist of curves of different factors in order 
to aid in interpreting or indicating their relation to each other. Curves of 
the same factor may be combined to form various series. The level series 
consists of all the level curves for the stations under observation, e. g., 
the six levels for temperature, three levels for wind, etc. Similarly, the 
station series is a combination of all the station curves, and a correspond- 
ing arrangement may be made for point curves with reference either to 
station or to level. An extremely valuable combination of curves is that of 
the holard and chresard for a series of stations. The most important com- 
binations of the curves of different factors are naturally those based upon 
factors intimately related to each other or to the plant. The grouping of 
water-content and humidity curves is of great value, especially when the 
transpiration curve is added. Light and temperature curves make an in- 
teresting combination, while a humidity, temperature, and wind series is of 
much aid in tracing the connection between these factors. Finally, it is 
altogether feasible to arrange the curves of water-content, humidity, light, 
temperature, and wind upon the same sheet in such fashion as to give a 
graphic representation of the whole physical nature of a single habitat or 
a series. In all combinations of curves representing different factors, it 
must be borne in mind that the position of a curve does not represent a 
definite value with reference to the others, since some are based upon per 
cents, others upon degrees, etc. The comparison must be based upon the 
character of the curves, but even then it is an important aid. An instruc- 
tive grouping has been employed where series of readings on the same day, 
or on two successive days in forest and in prairie have yielded the usual 
level series of curves. . The series for the two habitats are arranged on 
the same page, one at the right and the other at the left, and permit direct 



98 THE HABITAT 

comparison of corresponding level or factor curves, both with respect to 
position and character. 

142. The amplitude of all the curves described above is determined by 
the unit values of the factors concerned, while the length is dependent upon 
the number of stations, points, or times. The value assigned the latter 
upon the plotting paper is purely arbitrary, but it is most convenient to fix 
this at the centimeter square. The unit value for temperature is i° Centi- 
grade per square, each subdivision of the latter representing 0.2, and the 
range being 22 degrees. For water-content curves, each square represents 
a value of 2 per cent, the smaller square being 0.4 per cent, and the range 
2-48 per cent. The unit value for humidity is taken as 5 per cent, making 
each small square 1 per cent, and giving room on the sheet for the entire 
range from 1-100 per cent. Owing to the anemometer used, curves of 
wind velocity have been based upon the number of feet per minute. One 
hundred feet is taken as the unit value, and the range is from 0-2200 feet. 
The unit value for the curve of light intensity is .005. Each small square 
is .001, which permits a range from .001 to .01 on one sheet. Consequently, 
when it is desired to plot the curve of a series of habitats with a range in 
intensity greater than this it is necessary to use a double sheet. This is the 
usual device when the range of curves is too great, except where the excess 
is slight. In this case the curve is left open at the top, and the value which 
the crest attains is indicated. All curves in combination are labeled at the 
beginning or left to indicate the level, station, or point, and at the end or 
right to show the time, or day, if this is not the basis of the curve or series. 

The discussion that precedes deals exclusively with curves representing 
factors determined in the field. It applies with equal force to results 
obtained by instruments in control houses. In these, however, all factors 
except those directly experimented with, usually water-content and light, 
are practically equalized, and the curves based upon them are used chiefly 
to show how nearly equal they have become. The important curves are 
those of the water-content series, both holard and chresard, and of the 
shade tents. Where several houses are differentiated with respect to tem- 
perature or humidity, curve series of both these factors are necessary. 

Factor Means and Sums 

143. It has been shown elsewhere that the daily mean of temperature can 
be closely approximated from the maximum and minimum of both day and 
night. Maximum-minimum instruments for the other factors are lacking, 
however, and for light, humidity, and wind these values can only be ob- 



PHYSICAL FACTOR RESULTS 99 

tained from the ecograph which makes it possible to get the exact mean 
from the sum of all the hour readings. When it comes to the seasonal 
mean, the ecograph is even more necessary, exception being made for 
water-content, in' which case a number of readings on various days through 
the season will suffice. The value of factor means for diagnosis and for 
curves has already been sufficiently commented upon, and the feasibility of 
factor sums already indicated. 
LtfC. 



CHAPTER III. THE PLANT 

Stimulus and Response 
general relations 

144, The nature of stimuli. Whatever produces a change in the func- 
tions of a plant is a stimulus. The latter may be a force or a material ; it may 
be imponderable or ponderable ; effect, not character, determines a stimulus. 
Consequently, reaction or response decides what constitutes a stimulus. 
The presence of the latter can be recognized only through an appreciable 
or visible response, since it is impossible to discriminate between an impact 
which produces no reaction and one which produces a merely latent one. 
From this it is evident that quantity is decisive in determining whether the 
impact becomes a stimulus. Plants grow constantly under the influence 
of many stimuli, all varying from time to time in amount. Small changes 
in these are so frequent that, in many cases at least, the plant no longer 
appreciably reacts to them. Such changes, though usually measurable, are 
not stimuli. Futhermore, it must be clearly recognized that plants which 
are in constant response to stimuli are stimulated anew by an efficient in- 
crease or decrease in the amount of any one of these. As is well known, 
however, such increase or decrease is a stimulus only within certain limits, 
and the degree of change necessary to produce a response depends upon 
the amount of the factor normally present. The entire absence of a force 
usually present, moreover, often constitutes a stimulus, as is evident in the 
case of light. The nature of the plant itself has a profound bearing upon 
the factors that act as stimuli. Many species are extremely labile, and 
react strongly to relatively slight stimuli ; others are correspondingly stable, 
and respond only to stimuli of much greater force. Some light is thrown 
upon the nature of this difference by the behavior of ecads. A form which 
has grown under comparatively uniform conditions for a long time seems 
to respond less readily, and is therefore less labile than one which is sub- 
ject to constant fluctuation. In many cases this is not true, however, and 
the degree of stability, i. e., of response, can only be connected in a general 
way with taxonomic position. 

145. The kinds of stimuli. The factors of a habitat are external to 
the plant, and consequently are termed external stimuli. Properly speak- 
ing, all stimuli are external, but since the response is often delayed or can 



GENERAL RELATIONS IOI 

not be clearly traced, it may be permissible to speak of internal stimuli, i. e., 
those which appear to originate within the plant. These, however, are 
extremely obscure, and it is hardly possible to deal with them until much 
more is known of the action of external stimuli. Of the latter, certain 
forces, gravity and polarity, act in a way not at all understood, and as 
they are essentially alike for all plants and all habitats, they can here be 
ignored. Stimuli are imponderable when, like light and heat, they are 
measured with reference to intensity, and ponderable, when, as in the case 
of water-content, humidity, and salt-content, they can be expressed in mass 
or weight. It is undesirable to insist upon this distinction, however, since 
the real character of a stimulus is determined by its effect, and the latter 
is not necessarily dependent upon whether the stimulus is one of force or one 
of material. There is, however, a fundamental difference between factors 
with respect to their relation to the plant. Direct factors alone are stimuli, 
since indirect factors must always act through them. For example, the 
wind, its mechanical influence excepted, can affect the plant only in so far 
as it is converted into the stimulus of increased or decreased humidity. Con- 
sequently, the normal stimuli of the plants of a formation are : ( I ) water- 
content, (2) solutes, (3) humidity, (4) light, (5) temperature, (6) wind. 
Soil, pressure, physiography, and biotic factors influence plants only 
through these, and are not stimuli, though exceptions must be made of 
biotic factors in the case of sensitive, insectivorous, and gall-producing 
plants. 

146. The nature of response. Since plants have no special organs for 
the perception of stimuli, nor sensory tracts for their transmission, an ex- 
ternal stimulus acting upon a plant organ is ordinarily converted into a re- 
sponse at once. The latter as a rule becomes evident immediately; in some 
cases it is latent or imperceptible, or some time elapses before the chain of re- 
sponses finds visible expression. A marked decrease in humidity calls forth an 
immediate increase of transpiration, but the ultimate response is seen in the 
closing of the stomata. A response to decreased light intensity, on the 
other hand, is much less rapid and obvious. This difference is largely due 
to the fact that the functional response is more marked, or at least more 
perceptible in one case than in the other. 

Response is the reaction of the plant to a stimulus ; it begins with the 
impact of an efficient factor, and ends only with the consequent final read- 
justment. The immediate reaction is always functional. The nature and 
intensity of the stimulus determine whether this functional response is 
followed by a corresponding change in structure. The consideration of this 
theme consequently gains in clearness if a functional and a structural 



102 THE PLANT 

response be distinguished. The chief value of this distinction lies in the 
iact that many reactions are functional alone; it serves also to emphasize 
the absolute interdependence of structure and function, and the imperative 
need of considering both in connection with the common stimulus. For 
these reasons, the logical treatment is to connect each stimulus with its 
proper functional change, and, through this, with the corresponding modi- 
fication of structure. For the sake of convenience, the term adjustment 
is used to denote response in function, and adaptation, to indicate the re- 
sponse in structure. 

147. Adjustment and adaptation. The adjustment of a plant to the 
stimuli of its habitat is a constant process. It is the daily task, seen in 
nutrition and growth. So long as these take place under stimulation by 
factors which fall within the normal variation of the habitat, the problems 
belong to what has long been called physiology. When the stimuli become 
unusual in degree or in kind, by a change of habitat or a modification in it, 
adjustment is of much greater moment and is recorded in the plant's struc- 
ture. These structural records are the foundation of proper ecological 
study. Since they are the direct result of adjustment, however, this affords 
further evidence that a division of the field into ecology and physiology 
is illogical and superficial. Slight or periodical adjustment may concern 
function alone; it may be expressed in the movement of parts or organs, 
such as the closing of stomata or changes in the position of leaves, in 
growth, or in modifications of structure. This expression is fundamentally 
affected by the nature of the factor and is in direct relation to the intensity 
of the latter. Adaptation comprises all structural changes resulting from 
adjustment. It includes both growth and modification. The latter is 
merely growth in response to unusual stimuli, but this fact is the real 
clue to all evolution. Growth is periodic, and in a sense quantitative ; it 
results from the normal continuous adjustment of the plant to the stimuli 
of its proper habitat. In contrast, modification is relatively permanent and 
qualitative; it is the response to stimuli unusual in kind or intensity. A 
definite knowledge of the processes of growth is indispensable to an under- 
standing of modification. In the fundamental task of connecting plant and 
habitat, it is the modification of the plant, and not its growth, which records 
the significant responses to stimuli. For this reason the discussion of 
adaptation in the pages that follow is practically confined to modification 
of structure. This is particularly desirable, since growth has long been the 
theme of physiological study, while modification has too often been con- 
sidered from the structural standpoint alone. The comparatively few 
studies that have taken function into account have been largely empirical; 



GENERAL RELATIONS IO3 

in them neither stimulus nor adaptation has received anything approaching 
adequate treatment. 

148. The measurement of response. The amount of response to a 
stimulus is proportional to the intensity of the factor concerned. This does 
not mean that the same stimulus produces the same response in two distinct 
species, or necessarily in two plants of one species. In these cases the rule 
holds only when the plants or species are equally plastic. For each in- 
dividual, however, this quantitative correspondence of stimulus and response 
is fundamental. It is uncertain whether an exact or constant ratio can be 
established between factor and function ; the answer to this must await the 
general use of quantitative methods. There can be no doubt, however, 
that within certain limits the adjustment is proportional to the amount of 
stimulus, whereas reaction is well known to be abnormal or inhibited beyond 
certain extremes. It is quite erroneous to think that reaction is independent 
of quantity of stimulus, or to liken the stimulating factor to "the smallest 
spark (which) by igniting a mass of powder, produces an enormous 
mechanical effect." 1 Such a statement is only apparently true of the action 
of mechanical stimuli upon the few plants that may properly, be said to 
possess irritability, such as sensitive plants and certain insectivorous ones. 
Of the normal relation of response to direct factor's, water, light, etc., it is 
entirely untrue. Axiomatically, there is ordinarily an essential correspond- 
ence, also, between the amount of adjustment and of adaptation. This 
correspondence is profoundly affected, however, by the structural stability 
of the plant. 

From the preceding it follows that the measurement of response and the 
relating it to definite amounts of direct factors as stimuli are two of the 
most fundamental tasks of ecology. The exact determination of physical 
factors has no value apart from its use for this purpose. It is perfectly 
clear that precise methods of measuring stimuli call for similar methods in 
determining the amount of adjustment and of adaptation. The problem is- 
a difficult one, and it is possible at present only to indicate the direction 
which its development should take, and to describe a few methods which 
will at least serve as a beginning. To cover the ground adequately it is 
necessary to measure response by adjustment and by adaptation separately, 
and in the latter to find a measure for the individual and one for the 
species. The one is furnished by the methods of morphology and the other 
by biometry. 

1 Pfeffer-Ewart. Physiology of Plants, 1:13. 19C0. 






I04 THE PLANT 

A primary requisite for any method for measuring adjustment is that it 
be applicable to field conditions. Many instruments for measuring trans- 
piration, for example, are valueless, not because they are inaccurate, but 
because the plant studied is under abnormal conditions. To avoid the latter 
is absolutely necessary, a fact which makes it peculiarly difficult to devise a 
satisfactory field method. After the latter has been found and applied, it 
becomes possible to check other methods by it, and to give them real value. 
The final test of a field method is three-fold: (i) the plant must be studied 
while functioning normally in its own habitat; (2) the method must give 
accurate results; and (3) it must permit of extensive and fairly convenient 
application in the field. Until methods of this character, some of which 
are described later, have been employed for some time, it is impossible to 
connect definite intensities of factor stimuli with measured amounts of 
adjustment. Ultimately, it seems certain that researches will regularly take 
this form. 

Adaptation is primarily indicated by changes in the arrangement and 
character of the cells of the plant. Since these determine the form of each 
organ, morphology also furnishes important evidence in regard to the 
course of adaptation, but form can be connected certainly with adjustment 
only through the study of cellular adaptation. In tracing the modifications 
of cell and of tissue, the usual methods of histology, viz., sectioning and 
drawing, suffice for the individual. It is merely necessary to select plants 
and organs which are as nearly typical as can be determined. The ques- 
tion of quantity becomes paramount, however, since it often gives the clue 
to qualitative changes, and hence it is imperative that complete and accurate 
measurements of cells, tissues, and organs be made. These measurements, 
when extended to a sufficiently large number of plants, serve to indicate 
the direction of adaptation in the species. They constitute the materials for 
determining biometrically the mean of adaptation for the species and the 
probable evolution of the latter. In its present development, biometry con- 
tains too much mathematics, and too little biology. This has perhaps been 
unavoidable, but it is to be hoped that the future will bring about a wise 
sifting of methods, which will make biometry the ready and invaluable 
servant of all serious students of experimental evolution. This condition 
does not obtain at present, and in consequence it seems unwise to consider 
the subject of biometry in this treatise. 

149. Plasticity and fixity. As the product of accumulated responses, 
each species is characterized by a certain ability or inability to react to 
stimuli. Many facts seem to indicate that the degree of stability is con- 
nected with the length of time during which the species is acted upon by 



GENERAL RELATIONS 105 

the same stimuli. It seems probable that plants which have reacted to sun- 
light for hundreds of years will respond less readily to shade than those 
which have grown in the sun for a much shorter period. This hypothesis 
is not susceptible of proof in nature because it is ordinarily impossible to 
distinguish species upon the basis of the time during which they have 
occupied one habitat. Evidence and ultimate proof, perhaps, can be obtained 
only by field and control experiments, in which the time of occupation of 
any habitat is definitely known. Even in this case, however, it is clear 
that antecedent habitats will have left effects which can neither be traced nor 
ignored. Additional support is given this view by the fact that extreme 
types, both ecological and taxonomic, are the most stable. Intense xero- 
phytes and hydrophytes are much more fixed than mesophytes, though the 
intensity of the stimulus has doubtless as great an influence as its duration.' 
Composites, labiates, grasses, orchids, etc., are less plastic than ranals, 
rosals, etc., but there are many exceptions to the apparent rule that fixity- 
increases with taxonomic complexity. At present it seems quite impossible 
to suggest an explanation of the rule. Recent experiments indicate that 
there may be ancestral fixity of function, as well as of structure. It has 
been found, for example, that the flowers of certain species always react 
normally to the stimuli which produce opening and closing, while others 
make extremely erratic response. If further work confirms this result and 
extends it to other functions, the necessity of arriving at a better under- 
standing of fixity will be greatly emphasized. 

It is impossible to make progress in the study of adaptation without 
recog-nizing the fundamental importance of ancestral fixity as a factor. 
E. S. Clements 1 has shown that a number of species undergo pronounced 
changes in habitat without showing appreciable modification. Consequently, 
it is incorrect to assume that each habitat puts a structural impress upon 
every plant that enters it. For this reason, the writer feels that the current 
explanation of xerophytic bog plants, etc., is probably wrong, and that the 
discrepancy between the nature of the habitat and the structure of the plant 
is to be explained by the persistence of a fixed ancestral type. The anomaly 
is scarcely greater than in cases that have proved capable of being 
explained. 

150. The law of extremes. When a stimulus approaches either the 
maximum or minimum of the factor for the species concerned, response" 
becomes abnormal. The resulting modifications approach each other and 
in some respects at least become similar. Such effects are found chiefly in 

1 The Relation of Leaf Structure to Physical Factors. 1905. 



106 THE PLANT 

growth, but they occur to some degree in structure also. It is imperative 
that they be recognized in nature as well as in field and control experiment, 
since they directly affect the ratio between response and stimulus. The 
data which bear upon the similarity of response to extremes of different 
factors are too meager to permit the formulation of a rule. It is permissible, 
however, to suggest the general principle that extreme stimuli produce 
similar growth responses, and to emphasize the need of testing its appli- 
cation to adaptation proper. 

151. The method of working hypotheses. In the study of stimulus 
and response, where the unimpeachable facts are relatively few, and their 
present correlation slight, the working hypothesis is an indispensable aid. 
"The true course of inductive procedure . . . consists in anticipating 
nature, in the sense of forming hypotheses as to the laws which are prob- 
ably in operation, and then observing whether the combinations of 
phenomena are such as would follow from the laws supposed. The investi- 
gator begins with facts and ends with them. He uses such facts as are in 
the first place known to him in suggesting probable hypotheses ; deducing 
other facts which would happen if a particular hypothesis is true, he 
proceeds to test the truth of his notion by fresh observations or experi- 
ments. If any result prove different from what he expects, it leads him 
either to abandon or to modify his hypothesis ; but every new fact may give 
some new suggestion as to the laws in action. Even if the result in any 
case agrees with his anticipations, he does not regard it as finally confirma- 
tory of his theory, but proceeds to test the truth of the theory by new de- 
ductions and new trials." 1 In the treatment of adjustment and adaptation 
which follows, the method of multiple working hypotheses is uniformly em- 
ployed. No apology is felt to be necessary for this, since the whole endeavor 
is to indicate the proper points of attack, and not to distinguish between 
that which is conjectural and that which is known. If an hypothesis occa- 
sionally seem to be stated too strongly, it is merely that it appears, after a 
survey of the problem from all sides, to explain the facts most satisfactorily. 
The final proof of any hypothesis, however, rests not only upon its ability 
to explain all the facts, but also upon the inability of other hypotheses to 
meet the same test. The discovery and examination of all possible hypoth- 
eses, and the elimination of those that prove inadequate are the essential 
steps in the method of working hypotheses. 

ijEVONS, W. A. The Principles of Science, 2:137. 1874. 



GENERAL RELATIONS IO7 

HYDROHARMOSE 
ADJUSTMENT 

152. Water as a stimulus. Plants are continually subjected to the 
action of the water of the soil and of the air; exception must naturally 
be made of submerged plants. The stimulus of soil water acts upon the 
absorbing organ, the root, while that of humidity affects the part most 
exposed to the air, viz., the assimilative organ, which is normally the leaf. 
But since both are simultaneous water stimuli, a clearer conception is gained 
of this operation if they are viewed as two phases of the same stimulus. 
This point of view receives further warrant from the essential and intimate 
relation of humidity and water-content as determined by the plant. They 
are in fact largely compensatory, as is shown at some length later. In 
determining the intensity of the two, a significant difference between them 
must be recognized. The total humidity of the air at any one time consti- 
tutes a stimulus to the leaf which it touches. This is not true of the total 
soil water. Part of the latter is not available under any circumstances, 
and can not affect the plant, at least directly. The chresard alone can act 
as a stimulus, but even this is potential in the great majority of cases, since 
the actual stimulus is not the water available but the water absorbed. The 
latter, moreover, contains many nutrient salts which are in themselves 
stimuli, but as they normally have little bearing upon the action of water 
as a stimulus they are to be considered only when present in excessive 
amounts. 

153. The influence of other factors upon water. The amount of hu- 
midity is modified directly by temperature, wind, precipitation, and pressure, 
and, through these, it is affected by altitude, slope, exposure, and cover. 
Naturally, also, the evaporation of soil water has a marked influence. In 
determining water-content, atmospheric factors, with the exception of pre- 
cipitation, are usually subordinate to edaphic ones. Soil texture, slope, and 
precipitation act directly in determining soil water, while temperature, wind, 
and pressure can operate only through humidity. This is likewise true of 
altitude, exposure, and cover, though the latter has in addition a profound 
effect upon run-off. Biotic factors can affect humidity or water-content 
only through the medium of another factor. Light in itself has no action 
upon either, but through its conversion into heat within the chloroplast, it has 
a profound effect upon transpiration. The following table indicates the 
general relation between water and the other physical factors of •the habitat. 






MQ.8 the plant 



The order of the signs, ±, denotes, .that the water increases and decreases 
with an increase and decrease of the -factor, or the reverse, =F. 



Humidity =b 


Water-content zb 


Temperature =F 


Temperature =F 


Wind =F 


Wind =F 


Precipitation zb 


Precipitation zb 


Pressure zB.'-- 


Pressure =F 


Soil texture 


Soil texture 


Altitude =F 


Porosity =F 




Capillarity zb 


Slope =F 


Slope =F 


Exposure =F 


Exposure =F 


Cover zb 


• Cover zb 



154. Response. The normal functional responses to water stimuli are 
absorption, diffusion, transport, and transpiration. Of these, absorption and 
transpiration alone are the immediate response to soil water and humidity, 
respectively. Consequently they are the critical points of attack in study- 
ing the fundamental relation of the plant to the water of its habitat. In 
determining the pathway of the response, it is necessary to trace the steps 
in diffusion and transport, but, as these are essentially alike for all vascular 
plants, this task lies outside the scope of the work in hand. As previously 
suggested, the relation between absorption and transpiration is strictly com- 
pensatory, though, for obvious reasons, the amount of water transpired is 
usually somewhat less than the amount absorbed. Absorption falls below 
transpiration when extreme conditions cause temporary or permanent wilt- 
ing ; the two activities are essentially equal after a growing plant reaches 
maturity. In all cases, however, the. rule is that an increase or decrease in 
water loss produces a corresponding change in the amount of water 
absorbed, and, conversely, variation in absorption produces a consequent 
change in transpiration. This is strictly true only when the stimuli are 
normal. For example, a decrease in humidity causes increased water loss, 
which, through diffusion and transport, is compensated by increased activity 
of the root surface. Frequently the water supply is insufficient to compen- 
sate for a greater stimulus, and the proper balance can be attained only by 
the closing of the stomata. In the case of excessive stimuli, neither com- 
pensation suffices, and the plant dies. Many mesophytes and all xerophytes 
have probably resulted from stimuli which regularly approached the limit 
of compensation for each, and often overstepped, but never permanently 
exceeded it. For hydrophytes, the danger arises from excessive water 
supply, not water loss. There is a limit to the compensation afforded by 
transpiration, which is naturally dependent upon the amount of plant sur- 



HYDROHARMOSE TO9 

face exposed to the air. No compensation occurs in. the case of submerged 
plants ; floating hydrophytes possess a single transpiring leaf surface, while 
the leaves of amphibious plants behave as do those of mesophytes. The 
whole question of response to water stimuli thus turns upon the compensa- 
tion for water loss afforded by water supply where the latter is moderate 
or precarious, and upon the compensation for water supply furnished by 
water loss where the supply 'is excessive, submerged plants excepted. 

155. The measurement of absorption. As responses to measured 
stimuli of water-content and humidity, it is imperative that the amount of 
absorption and of transpiration be determined quantitatively. It is also 
extremely desirable that this be done in the normal habitat of the plant. 
A careful examination of the problems to be met quickly discloses the great 
difficulty of obtaining a direct and accurate measure of absorption under 
normal conditions, especially in the field. For this purpose, the ordinary 
potometric experiments by means of cut stems are valueless. The use of 
the entire plant in a potometer yields much more trustworthy results, though 
the fact that the root is under abnormal conditions can not be overlooked, 
especially in the case of mesophytes and xerophytes. "While potometric 
conditions are less abnormal for amphibious plants, the error is not wholly 
eliminated, since the roots normally grow in the soil. The potometer can 
be made of value for quantitative work only by checking the results it gives 
by means of an instrument or a method in which the plant functions nor- 
mally. In consequence, the potometer can not at present be used to measure 
absorption directly, though, as is further indicated in the discussion of 
transpiration, it is a valuable supplementary instrument, after the check 
mentioned has been applied to its use with a particular species. 

An estimate of the amount of absorption may be obtained either in the 
field or in the control house by taking samples from the protected soil at 
different times. Since it is impossible to determine the weight of the area 
in which the roots lie, and since the soil water is often unequally distributed, 
this method can not yield exact results. An accurate method of measuring 
absorption under essentially normal conditions has been devised and tested 
in the control house. The essential feature of the process is the placing a 
plant in a soil containing a known quantity of water, and removing it after 
it has absorbed water from the soil for a certain period. In carrying out 
the experiment, a soil consisting of two parts of sod and one of sand was used, 
since the aeration is more perfect and the particles are more easily removed 
from the roots. The soil was completely dried out in a water bath and then 
placed in a five-inch battery jar. The latter, together with the rubber cloth 
used later to prevent evaporation, was weighed to the decigram. A 
weighed quantity of water was added, and the whole again weighed as a 



no 



THE PLANT 



check. Two plants of Helianthus annuus were taken from the pots in which 
they had grown, and the soil was carefully washed from the roots. Each 
plant was weighed with its roots in a dish of water to prevent wilting, 
and then carefully potted, one in each battery jar. A thistle tube was placed 
in the soil of each jar -to facilitate aeration, as well as the addition of 
weighed amounts of water, when necessary, and the rubber cloth attached 
in the usual manner to prevent evaporation. The entire outfit was weighed 
again, and the weighing repeated at 8:00 a.m. and 5:00 p.m. for five days, 
in order to determine the amount of transpiration and its relation to the 
water absorbed. The plants were kept in diffuse light to prevent excessive 
water loss while the roots were becoming established. At the close of the 
experiment, the jar and its contents were weighed finally. The plants were 
removed and weighed, the soil particles being shaken from the roots into 
the jar, which was also weighed. The results obtained were as follows : 



I 

II 



Wt. of pot 
and dry soil 



1846.0 

1886.7 



Wt. of pot and wet soil 



1 
2218.0 g. 
2253.2 g. 



11 
2174.3 g. 
2221.6 g. 



Total H2O 



372.0 g. 
366.5 g. 



mo left 



328.3 g. 
334.9 g. 



mo ab- 
sorbed 



43.7 g. 
31.6 g. 



mO tran- 
spired 



43.7 
31.6 



The amount of water absorbed may be obtained directly by subtracting the 
final weight of the jar and moist soil from their first weight, but a desirable 
check is obtained by taking the dry weight of jar and soil from the first, 
and the final weight of these, and subtracting the one from the other as 
indicated in the table. A second check is afforded by daily weighings, 
from which the amount of water transpired is determined. Since the two 
sunflower plants made practically no growth during the period of experi- 
ment, the exact correspondence between water absorbed and water lost is 
not startling, though it can not be expected that the results will always 
coincide. 

This method has certain slight sources of error, all of which, it is thought, 
have been corrected in a new and more complete series of experiments 
now being carried on. The aeration of the soil is not entirely normal, as is 
also true of the capillary movements of the water, on account of the non- 
porous glass jar and the rubber cloth. Since the latter are necessary condi- 
tions of all accurate methods for measuring absorption and transpiration, 
the resulting error must be ignored. It can be reduced, however, by forcing 
air through the thistle tube from time to time. Sturdy plants, such as the 
sunflower, are the most satisfactory, since they recover more quickly from 
the shock of transplanting. Almost any plant can be used, however, if 



HYDROHARMOSE 



III 



repotted in a loose sandy soil often enough. This permits the root system 
to develop normally, and also makes it possible to wash the soil away with- 
out injury to the root. The method is so recent that there has been 
no opportunity to test it in the field. It would seem that it can be applied 
without essential change to plants in their normal habitats. Very large 
herbs or plants with extensive root systems could not be used to advantage, 
and to be practicable the experiments would need to be carried on near the 
base station. The great value of the method, however, lies in its use as a 
check in determining the accuracy of other methods, and in practice it will 




Fig. 31. Absorption and transpiration of Helianthus annuus. land 
II, plants repotted in soil of known weight and water-content; III, 
plant undistured in the original soil; IV, potometer containing plant 
with cut stem; V, potometer with entire plant. 

often be found convenient and time-saving to use the latter, after they have 
once been carefully checked for different groups of species. This matter is 
farther considered under measures of transpiration. 

156. The quantitative relation of absorption and transpiration. Bur- 

gerstein 1 has summarized the results of various investigators in the state- 
ment "that between the quantitative absorption of water on the one hand 
and emission on the other there exists no constant parallelism or proportion,'' 



1 Die Transpiration der Pflanzen, 14. 1904. 



112 THE PLANT 

and he has cited the work of Krober, and of Eberdt in proof. This state- 
ment holds, however, only for short periods of a few hours, or more rarely, a 
day, and even here its truth still remains to be conclusively demonstrated. 
The discrepancy between absorption and transpiration for a short period is 
often greater than for a longer time, but it is evident that a transient change 
in behavior or a small error in the method would inevitably produce this 
result. Eberdt found the discrepancy for a few hours to be 1-2 ccm. in an 
entire plant of Helianthus annuus, while for a whole day the water absorbed 
was 33-57 ccm - an d the water lost 33.98 ccm. Krober's experiments with 
cut branches of Asclepias incarnate showed a maximum difference for 12 
hours of 2.5 ccm., but the discrepancy for the first 24 hours was 1 ccm. 
and for the second 1.9 ccm. In both cases, the potometer was employed. 
Consequently, as will be shown later, Eberdt's results are not entirely trust- 
worthy, while those of Krober, made with cut stems, are altogether unre- 
liable. Kence, it is clear that the discrepancy is slight for a period of several 
days or weeks, and that it may be ignored without serious error, except in 
a few plants that retain considerable water as cell sap, in consequence of 
extremely rapid growth. Accordingly, the amount of transpiration, which 
may be readily and accurately determined, can be employed as a measure 
of absorption that is sufficiently accurate for nearly all purposes. The 
truth of this statement may be easily confirmed. It is evident that the 
amount of water absorbed equals the amount transpired plus that retained 
by the plant as cell-sap, or used in the manufacture of organic compounds. 
In plants not actively growing, the amount lost equals that absorbed, as 
already shown in the experiment with Helianthus. According to Gain 1 , 
Deherain has found that a plant rooted in ordinary soil transpired 680 kg. 
of water for each kilogram of dry substance elaborated. In Helianthus 
annuus, the dry matter is 10 per cent of the weight of the green plant. A 
well-grown plant weighing 1,000 grams, therefore, consists of 100 grams 
of dry matter and 900 of water. The length of the growing period for 
such a plant is approximately 100 days, during which it transpires 68 kilo- 
grams of water. Assuming the rate of transpiration <and of growth to be 
constant, the plant transpires 680 grams daily, adds 9 grams to its cell-sap, 
and 1 gram to its dry weight. The amount of water in a gram of cellulose 
and its isomers is about 3/5. Consequently, the total water absorbed daily 
by the plant is 689.6 grams. The 680 grams transpired are 98.6 per cent 
of the amount absorbed; in other words, only 1.4 per cent of the water 
absorbed is retained by the plant. From this it is evident that the simplest 

iRecherches sur le Role Physiologique de l'Eau dans la Vegetation. Ann. Nat. 
Sci., 7:20:65. 1895. 



HYDROHARMOSE II3 

and most convenient measure of absorption under normal conditions can be 
obtained through transpiration, since the discrepancy between absorption and 
transpiration is scarcely larger than the error of any method applicable 
to the field. Conversely, the measure of absorption obtained by the process 
described in the preceding section serves also as a measure of transpiration. 
The determination of the latter in the field is so much simpler, however, 
that it is rarely desirable to apply the absorption method. 

157. Measurement of transpiration. The water loss of a plant may be 
determined absolutely or relatively. Absolute or quantitative determinations 
are by (i) weighing, (2) collecting, or (3) measuring the water absorbed; 
relative values are indicated by hygroscopic substances. A number of 
methods have been employed more or less generally for measuring trans- 
piration. The great majority of these can be used to advantage only in the 
laboratory, and practically all fail to meet the fundamental requirement for 
successful field work, namely, that the plant be studied under normal con- 
ditions in its own habitat. The following is a summary of the various 
methods, the details of which may be found in Burgerstein. 

1. Weighing. This is the most satisfactory of all methods for deter- 
mining water loss. It is more accurate than any other, and is unique in that 
it does not place the plant under abnormal conditions. On the score of 
convenience, moreover, it excels every other method capable of yielding 
quantitative results. Various modifications of weighing are employed, but 
none of these have all the advantages of a direct, simple weighing of the 
plant in its own soil. 

2. Collecting the zcater transpired. This may be done by collecting 
and weighing the water vapor exhaled by a plant placed within a bell jar, 
or by weighing a deliquescent salt, such as calcium chloride, which is used 
to absorb the water of transpiration. The decisive disadvantage of these 
methods is that transpiration is carried on in an atmosphere far more humid 
than normal. If an excessive amount of salt is used, the air is abnormally 
dry. In both cases, the water loss decreases until it reaches a point much 
below the usual amount. Finally, all methods of this kind are open to con- 
siderable error, and are inconvenient, especially in field work. They aie 
of relatively slight value in comparison with weighing. 

3. Potometers. It has already been shown that the amount of water 
absorbed is a close measure of the amount transpired. In consequence, the 
potometer can be used to determine the amount of transpiration provided 
the absorption- is not abnormal. It is rarely and only with much difficulty 
that this condition can be met. The use of cut stems and branches does not 
meet it, and even in the case of plants with roots, the results must be 



114, THE PLANT 

compared with those obtained from absorption experiments made with 
plants rooted in soil before they can be relied upon. This necessity practi- 
cally puts the potometer out of commission for accurate work, unless future 
study may show a somewhat constant ratio between the absorption of a plant 
in its own soil and that of a plant placed in a potometer. 

4. Measuring absolute humidity. The cog psychrometer makes it 
possible to determine the increased relative humidity produced within a 
glass cylinder or special tin chamber by a transpiring plant. From this 
result the absolute humidity is readily obtained, and by means of the latter 
the actual amount of water given off. The evident drawback to this 
method is that the increasing humidity within the chamber gives results 
entirely abnormal for the plant concerned. 

5. Self-registering instruments. There are various methods for regis- 
tering the amount of transpiration, based upon weighing', or upon the poto- 
meter. The Richard recording evaporimeter has all the advantages of 
weighing, inasmuch as the water loss is measured in this way, and in addi- 
tion the amount is recorded upon a revolving drum, obviating the necessity 
of repeated attention in case it is desirable to know the exact course of 
transpiration. On the other hand, methods which depend upon the poto- 
meter, while graphic, are not sufficiently accurate to be of value. 

6. The use of hygroscopic materials. Hygroscopic substances change 
their form or color in response to moisture. As they indicate comparative 
water loss alone, they are of value chiefly in the study of the stomatic 
surfaces of leaves. F. Darwin 1 has used strips of horn, awns of Stipa, and 
epidermis of Yucca to construct small hygroscopes for this purpose. In 
these instruments the error is large, but as no endeavor is made to obtain 
exact results, it is negligible. Filter paper impregnated with a 3-5 per 
cent aqueous solution of cobalt chloride is deep blue when dry. If a strip 
of cobalt paper is placed upon a leaf and covered with a glass slip it turns 
bright rose color, the rapidity of the change affording a clue to the amount 
of transpiration. 

158. Field methods. The conditions which a satisfactory field method 
of measuring transpiration must fulfill have already been discussed; they 
are accuracy, simplicity, and normality. These conditions are met only by 
weighing the plant in its own soil and habitat. This has been accomplished 
by- means of the sheet-iron soil box, already described under the determina- 
tion of the chresard. The method is merely the familiar one of pot and 
balance, slightly modified for field use. The soil block, which contains the 

: l Observations on Stomata by a New Method. Proc. Camb. Phil. Soc, 9:303. 1897. 



HYDROHARMOSE 115 

plant to be studied, is cut out, and the metal plates put in position as indi- 
cated in section 53. Indeed, it is a great saving of time and effort to deter- 
mine transpiration and chresard in the same experiment ; this is particu- 
larly desirable in view of the close connection between them. In this event, 
the soil block must be small enough not to exceed the load of a field balance. 
After the block is cut and encased, all the plants are removed, except the 
one to be studied. If several individuals of the same species are present, 
it is an advantage to leave all of them, since the error arising from individual 
variations of water loss may, in this way-, be almost completely eliminated. 
A sheet of rubber or rubber cloth is carefully tied over the box to prevent 
evaporation from the soil. A broad band is passed under the box to aid in 
lifting it upon the scales. The latter must be of the platform type, and 
should have a capacity as great as consistent with the need for moving 
it about in the field. Weighings are made in the usual way, care being 
taken to free the surface of the box from soil. The aeration of the soil block 
is kept normal by removing the rubber for a few minutes from time to 
time, or by forcing air through a thistle tube. Water is also added through 
the latter, when it is desired to continue the experiment for a considerable 
period. After the study of transpiration is concluded, the rubber cloth is 
removed, soil samples taken, and the soil allowed to dry out 'until the plant 
becomes thoroughly wilted. If the box is weighed again, the difference 
represents the amount of available water. The per cent of chresard is also 
obtained in the usual way by taking samples for ascertaining the echard, 
and subtracting* this from the holard. Field determinations of water loss 
yield the most valuable results when different habitat forms, or ecads, of 
the same species are used. There is little profit in comparing the transpira- 
tion of a typical sun plant, such as Touterea multiflora, with that of a shade 
plant, such as JVashingtonia obtusct. But the simultaneous study of plants 
like Chamacnerium angustifolium, Gentiana acuta, Scutellaria brittouii 
etc., which grow in several different habitats, furnishes direct and funda- 
mental evidence of the course of adjustment and adaptation. 

Hesselmann 1 , in his study of open woodlands in Sweden, has employed 
a method essentially similar to the preceding. Young plants of various 
species were transferred to pots in the field, where they were allowed to 
grow for several months before a series of weighings was made to determine 
the amount of transpiration. Since weighing is the measure used in each, 
both methods are equally accurate. The one has a certain advantage in that 
the pots are, perhaps, more easily handled, while the other has the advantage 

1 Z\iv Kenntnis des Pnanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 
18:311. 1904. 



n6 



THE PLANT 



of maintaining the normal relation of soil and roots, a condition more or less 
impossible in a pot. In both instances the weighing should be done in the 
habitat, which was not the case in Hesselmann's researches. 

The slight value of the potometer, which has had a vogue far beyond its 
merits, is indicated by the following table. These results were obtained 
from three plants of Helianthus animus; III was left undisturbed in the 
pot where it had been growing, IV was placed in a potometer, after the 
root had been cut off, and V was an entire plant placed in a potometer. 
The amount of transpiration is indicated in grams per square decimeter of 
leaf surface. The plants were kept in diffuse light, except for a period of 
two hours (8:00 to 10:00 a.m.) on the last day, when they were in full 
sunshine at a temperature of 75 ° F. Plant IV w r ilted so promptly in the 
sunshine that it was found necessary to conclude the experiment in 
diffuse light. 





8 A.M. 


5 p.m. 


8 A.M. 


5 F.M. 


8 a.m. 


5 p.m. 


8 a.m. 


10a.m. 


5 P.M. 


8 a.m. 


Total 


Ill 

IV 

V 


2.9 
4.7 
3.7 


7.3 
7.2 
5.3 


2.4 
2.9 
3.2 


6.0 
2.3 

4.8 


1.7 
1.0 

2.5 


1.6 
0.6 
1.6 


2.0 
0.9 
3.0 


3.4 
0.5 
2.6 


2.0 
0.5 
1.6 


1.8 
0.4 
2.6 


31.1 
21.0 
30.9 



The cut plant, IV, lost more water the first day than either of the others, 
but the water loss soon decreased, and at the end of the period was almost 
nil. The total transpiration for III and V is much the same, but the range 
of variation for periods of 12 hours is from -\-2 to — 1 gram. This ex- 
periment is taken as a fair warrant that the use of cut stems in "potometers 
can not give accurate results. It is inconclusive, however, as to the merits of 
potoinetric values obtained by means of the entire plant, and further studies 
are now being made with reference to this point. 



159. Expression of results. From the previous discussion of the relation 
between them, it follows that an expression of the amount of transpiration 
likewise constitutes an expression of absorption. It is very desirable also 
that the latter be based upon root surface and chresard, but the difficulty of 
determining the former accurately and readily is at present too great to 
make such a basis practicable. In expressing transpiration in exact terms, 
the fact that plants of the same species or form are somewhat individual in 
their behavior must be constantly reckoned with. In consequence, experi- 
ments should be made upon two or three individuals whenever possible, in 
order to avoid the error arising from this source. 



H YDROH ARMOSE 1 1 7 

Water loss may be expressed either in terms of transpiring surface or 
of dry weight. Since there is no constant relation between surface and 
weight, the terms are not interchangeable or comparable, and in practice it 
is necessary to use one to the exclusion of the other. Obviously, surface 
furnishes by far the best basis, on account of its intimate connection with 
stomata and air-spaces, a conclusion which Burgerstein (/. c, p. 6) has 
shown by experiment to be true. For the best results, the whole transpiring 
surface should be determined. This is especially necessary in making com- 
parisons of different species. In those studies which are of the greatest 
value, viz., ecads of the same species, it is scarcely desirable to measure stem 
and petiole surfaces, unless these organs show unusual modification. The 
actual transpiring surface is constituted by the walls of the cells bordering 
the intercellular spaces, but, since it is impossible to determine the aggregate 
area of these, or the humidity of the air-spaces themselves, the leaf surface 
must be taken as a basis. Since the transpiration through the stomata is 
much greater than that through the epidermal walls, the number of stomata 
must be taken into account. Since they are usually less abundant on the 
upper surface, their number should be determined for both sides of the 
leaf. The errors arising from more or less irregular distribution are elimi- 
nated by making counts near the tip, base, and middle of two or three ma- 
ture leaves. The most convenient unit of leaf surface is the square deci- 
meter. The simplest way to determine the total leaf area of a plant is to 
outline the leaves upon a homogeneous paper, or to print them upon a 
photographic paper. The outlines are then cut out and weighed, and the 
leaf area obtained in square decimeters by dividing the total weight by the 
weight of a square decimeter of the paper used. The area may also be 
readily determined by means of a planimeter. 

160. Coefficient of transpiration. At present it does not seem feasible 

to express the transpiration of a plant in the form of a definite coefficient, 

but ii is probable that the application of exact methods to each part of the 

problem will finally bring about this result. Meanwhile the following 

u 
formula is suggested as a step toward this goal: t=g—, LHT, in which 

t, the transpiration relation of a plant, is expressed by the number of grams 
of water lost per hour, on a day of sunshine, by one square decimeter of 
leaf, considered with reference to the stomata of the two surfaces, and the 
amount of the controlling physical factors, light, humidity, and temperature, 
at the time of determination. For Helianthus animus, this formula would 

appear as follows: /=2 : 1:50:75°. To avoid the large figures arising 

250 






Il8 THE PLANT 

from the extent of surface considered, the number of stomata per 
square decimeter is divided by 10,000. This amounts to the number per 
square millimeter, and time may consequently be saved by using this figure 
directly. While this formula obviously leaves much to be desired, it has 
the great advantage of making it possible to compare ecads of one species, 
or species of the same habitat or of different habitats, upon an exact basis 
of factor, function, and structure. 

ADAPTATION 

/ 

161. Modifications due to water stimuli. In adaptation, the great 
desideratum is to connect each modification quantitatively with the corres- 
ponding adjustment. This is even more difficult than to ascertain the 
quantitative relation between stimulus and functional response, a task still 
beset with serious obstacles. At the present time, little more can be done 
than to indicate the relation of marked adaptations of organs and tissues to 
the direct factors operating upon them, and to attempt to point out among 
the functions possibly concerned the one which seems to be the most prob- 
able connection between the probable stimulus and the structure under 
investigation. In the pages that follow, no more than this is attempted. 
The general changes of organs and tissues produced by water are first dis- 
cussed, and after this is given a summary of the structural features of the 
plant types based upon water-content. 

162. Modifications due to a small water supply. A water supply which 
may become deficient at any time is compensated either by changes which 
decrease transpiration, or by those that increase the amount of water 
absorbed or stored. These operate upon the form and size of the organs 
concerned, as well as upon their structure. Modifications of the form of 
leaf and stem are alike in that they lessen transpiration by a reduction of 
the amount of surface exposed to the air. Structural adaptations, on the 
other hand, bring about the protection of epidermal cells and stomata, and 
often internal cells also, from the factors which cause transpiration, or 
they anticipate periods of excessive transpiration by the storage of water 
in specialized cells or tissues. In certain extreme types the epidermis is 
itself modified for the absorption of water vapor from the air. 

163. The decrease of water loss. The following is a summary of the 
contrivances for reducing transpiration. 

i. Position of the leaf. Since the energy of a ray of sunlight is greatest 
at the sun's highest altitudes, those leaves transpire least which are in such a 



HYDROHARMOSE 119 

position during midday that the rays strike them as obliquely as possible. A 
leaf at right angles to the noonday sun receives ten times as much light 
and heat upon a square decimeter of surface as does one placed at an angle 
of 10 degrees. This device for reducing the intensity of insolation is best 
developed in the erect or hanging leaves of many tropical trees. In tem- 
perate zones, it is found in such plants as Silphium laciniatum and Lactuca 
scariola, and in species with equitant leaves. In such plants as Helianthus 
a 11 iiu us, the effect is just the opposite, since the turning of the crown keeps 
the leaves for a long time at a high angle to the incident rays. In the case 
of mats, it is the aggregation of plants which brings about the mutual pro- 
tection of the leaves from insolation and wind. 

2. Rolling of the leaf. Many grasses and ericaceous plants possess leaves 
capable of rolling or folding themselves together when drouth threatens. 
In other cases, the leaves are permanently rolled or folded. The advantage 
of this device arises not only from the reduction of surface, but also from 
the fact that the stomata come to lie in a chamber more or less completely 
closed. In the case of those mosses whose leaves roll or twist, a reduction 
of surface alone is effected. 

3. Reduction of leaf. The transpiring surface of a plant is reduced by 
decreasing the number of leaves, by reducing the size of each leaf, or by a 
change in its form. In so far as the stem is a leaf, a decrease in size or a 
change in shape brings about the same result. The final outcome of reduc- 
tion in size or number is the complete loss of leaves, and more rarely, of 
the stem. Such marked decrease of leaf area is found only in intense 
xerophytes, though it occurs in all deciduous trees as a temporary adapta- 
tion. Changes in leaf form are nearly always accompanied by a decrease 
in size. Of the forms which result, the scale, the linear or cylindrical leaf, 
and the succulent leaf are the most common. Leaves which show a 
tendency to divide often increase the number of lobes or make them smaller. 

4. Epidermal modifications. Excretions of wax and lime by the epidermis 
have a pronounced effect by increasing the impermeability of the cuticle, 
and, hence, decreasing epidermal transpiration. It seems improbable that 
a coating of wax on the lower surface of a diphotic leaf can have this 
purpose. The thickening of the outer wall of epidermal cells to form a 
cuticle is the most perfect of all contrivances for decreasing permeability 
and reducing transpiration. In many desert plants, the greatly thickened 
cuticle effectually prevents epidermal transpiration. In these also the 
cuticle is regularly developed in such a way as to protect the guard cells, 
and even to close the opening partially. An epidermis consisting of two or 
more; layers of cells is an effective, though less frequent device against water 
loss. When combined with a cuticle, as is usually the case, the imperme- 
ability is almost complete. Hairs decrease transpiration by screening the 



120 THE PLANT 

epidermis so that the amount of light and heat is diminished, and the access 
and movement of dry air impeded. While hairs assume the most various 
forms, all hairy coverings serve the same purpose, even when, as in the 
case of Mcscmbryanthcinum, they are primarily for water-storage. Hairs 
protect stomata as well as epidermal cells : the greater number of the former 
on the lower surface readily explains the occurrence of a hairy covering on 
this surface, even though absent on the more exposed upper side. In some 
cases, hairs are developed only where they serve to screen the stomata. 

The modifications of the stomata with respect to transpiration are numer- 
ous, yet all may be classed with reference to changes of number or level. 
With the exception -of aquatic and some shade plants, the number of stomata 
is normally greater on the less exposed, i. e., lower surface. The number 
on both surfaces decreases regularly as the danger of excessive water loss 
increases, but the decrease is usually more rapid on the upper surface, which 
finally loses its stomata entirely. It has been shown by many observers that 
species growing in dry places have fewer stomata to the same area than do 
those found in moist habitats. This result has been verified experimentally 
by the writer in the case of Ranunculus sceleratus, in which, however, the 
upper surface possesses the larger number of stomata. Plants of this 
species, which normally grow on wet banks, were grown in water so 
that the leaves floated, and in soils containing approximately 10, 15, 30, 
and 40 per cent of water. The averages for the respective forms were: 
upper 20, lower o; upper 18, lower 10; upper 18, lower 11; upper 11, 
lower 8; upper 10, lower 6. Reduction of number is effective, however, 
only under moderate conditions of dryness. As the latter becomes intense, 
the guard cells are sunken below the epidermis, either singly or in groups. 
In both cases, the protection is the same, the guard cells and the opening 
between them being withdrawn from the intense insolation and the dry air. 
The sun rays penetrate the chimney-shaped chambers of sunken stomata 
only for a few minutes each day, and they are practically excluded from the 
stomatal hollows which are filled with hairs. The influence of dry winds is 
very greatly diminished, as is also true, though to a less degree, for leaves 
in which the stomata are arranged in furrows. Sunken stomata often have 
valve-like projections of cuticle which reduce the opening also. Finally, in 
a few plants, water loss in times of drouth is almost completely prevented 
by closing the opening with a wax excretion. 

5. Modifications in the chlorenchym. A decrease in the size and number 
of the air passages in the leaf renders the movement of water-laden air to 
the stomata more difficult, and effects a corresponding decrease in transpi- 
ration. The increase of palisade tissue, though primarily dependent upon 
light, reduces the air-spaces, and consequently the amount of water lost. 



H YDROHARMOSE I 2 1 

The development of sclereids below the epidermis likewise hinders the 
escape of water. Finally, the character of the cell sap often plays an im- 
portant part, since cells with high salt-content or those containing mucila- 
ginous substances give up their water with reluctance. 

164. The increase of water supply. Plants of dry habitats can increase 
their absorption only by modifying the root system so that the absorbing 
surfaces are carried into the deep-seated layers of soil, and the surfaces in 
contact with the dry soil are protected by means of a cortex. Exception 
must be made for epiphytes and a few other plants that absorb rain water 
and dew through their leaves, and for those desert plants that seem to con- 
dense the moisture of the air by means of hygroscopic salts, and absorb it 
through the epidermis of the leaf. The storage of water in the leaf is a 
very important device; it increases the water supply by storing the surplus 
of absorbed water against the time of need. Modifications for water 
storage are occasionally found in roots and stems, but their chief develop- 
ment takes place in the leaf. The epidermis frequently serves as a reser- 
voir for water, either by the use of the epidermal cells themselves, by the 
formation of hypodermal water layers, or by means of superficial bulliform 
cells. The water cells of the chlorenchym regularly appear in the form of 
large clear cells, scattered singly or arranged in groups. In this event, 
they occur either as transverse bands, or as horizontal layers, lying between 
the palisade and sponge areas, and connecting the bundles. A few plants 
possess tracheid-like cells which also serve to store water. In the case of 
succulent leaves, practically the whole chlorenchym is used for storing water, 
though they owe their ability to withstand transpiration to a combination 
of factors. 

165. Modifications due to an excessive water supply. Water plants 
with aerial leaf surfaces are modified in such manner as to increase water 
loss and to decrease water supply, but the resulting modifications are 
rarely striking. There is a marked tendency to increase the exposed surface. 
This is indicated by the fact that, while the leaves of mud and floating forms 
become larger, they change little or not at all in thickness. The lobing of 
leaves is also greatly reduced, or the lobes come to overlap. Leaves of 
water plants are practically destitute of all modifications of epidermis and 
stomata, which could serve to hinder transpiration. The stomata are 
usually more numerous on the upper surface, and in the same species their 
number is greater in the forms grown in wet places. These facts explain 
in part the extreme development of air-passages in water plants, though this 
is, in large measure, a response to the increasing difficulty of aeration. The 






122 



THE PLANT 



increase of air-spaces is correlated with reduction of the palisade, and a 
decided increase in the sponge. An increase in water supply is indicated by 
the absence of storage tissues, and the reduction of the vascular system, 
which, however, is more closely connected with a diminished need for 
mechanical support. 

166. Plant types. The necessity for decreasing or increasing water loss 
in compensation of the water supply has made it possible to distinguish two 
fundamental groups of plants upon the twofold basis of habitat and struc- 
ture. These familiar groups, xerophytes and hydrophytes, represent two 
extremes of habitat and structure, between which lies a more or less vague, 
intermediate condition represented by mesophytes. These show no char- 
acteristic modifications, and it is consequently impossible to arrange them 
in subgroups. Xerophytes and hydrophytes, on the other hand, exhibit 
marked diversity among themselves, a fact that makes it desirable to 

recognize subgroups, which 
correspond to fundamental 
differences of habitat or 
adaptation. It is hardly neces- 
sary to point out that these 
types are not sharply defined, 
or that a single plastic species 
may be so modified as to ex- 
hibit several of them. The 
extremes are always clearly 
defined, however, and they 




Fig. 32. Mesophyll of Pedicularis procera 
(chresard, 15$, light, 1). X 130. 



indicate the specific tendency of the adaptation shown by other members 
of the same group. 

167. Xerophytic types. With the exception of dissophytes, all xero- 
phytes agree in the possession of a deep-seated root system, adapted to 
withdraw' water from the lower moist layers, and to conserve from loss 
from the upper dry layers. Reservoirs are developed in the root, however, 
in relatively few cases. The stem follows the leaf more or less closely in 
its modification, except when the leaf is greatly reduced or disappears, in 
which event the stem exhibits peculiar adaptations. While the leaf is by 
far the most strikingly modified, it is a difficult task to employ it satisfactorily 
as the basis for distinguishing types. Several adaptations are often com- 
bined in the same leaf, and it is only where one of these is preeminently 
developed, as in the case of succulence, that the plant can be referred to a 
definite type. The latter does not happen in many species of the less 



HYDROHARMOSE 



123 



intensely xerophytic habitats, and, consequently, it is difficult, if not unde- 
sirable, to place such xerophytes under a particular group. The best that 
can be done is to recognize the types arising from extreme or characteristic 
modification, and to connect the less marked forms as closely as possible 
with these. Halophytes differ from xerophytes only in the fact that the 
chresard is determined by the salt-content of the habitat, and not by the tex- 
ture of the soil. In consequence, they should not be treated as a distinct 
group. 

168. Types of leaf xerophytes. 
In these, adaptation has acted 
primarily upon the leaf, while 
the stem has remained normal for 
the most part. Even when the 
leaves have become scale-like, 
they persist throughout the grow- 
ing season, and continue to play 
the primary part in photosyn- 
thesis. The following types may 
be distinguished : 

1. The normal form. The 
leaf is of the usual dorsiventral 
character. In place of a reduc- 
tion in size, structural modifica- 
tions are used to decrease 
transpiration. With respect to 
the protective feature that is 
predominant, three subtypes may 
be recognized. The cntinized leaf 
compensates for a low water-con- 
tent by means of a thick cuticle, 
often reinforced by a high de- 
velopment of palisade tissue. 
Such leaves are more or less 
leathery, and they are often evergreen also. Arctostaphylus and many 
species of Pentstemon are good examples. Lanate leaves, i. e., those with 
dense hairy coverings on one or both surfaces, as Artemisia, Antennaria, 
etc., regularly lack both cuticle and -palisade tissue. The protection against 
water loss, however, is so perfect that the chlorenchym often assumes the 
loose structure of a shade leaf. Storage leaves usually have a well- 
developed cuticle and several rows of palisade cells, but their characteristic 
feature is the water-storage tissue, which maintains a reserve supply of 




Fig. 33. Staurophyll of Bahia dissecta, 
showing extreme development of palisade 
(chresard, 3-9$; light, 1). X 130. 



124 



THE PLANT 



water for the time of extreme drouth. Xerophytic species of Helianthus 
furnish examples of transverse bundles of storage cells, while those of 
Mertensia illustrate the more frequent arrangement in which the water tissue 
forms horizontal layers. 

2. The succulent form. Many succulent leaves are normal in shape and 
size, though always thicker than ordinary leaves. Usually, however, they 
are reduced in size and are more or less cylindrical in form. The necessary 
decrease in transpiration is effected by the reduction in surface, the general 
storage of water, a waxy coating, and, often also, by a very thick cuticle. 
Agave, Mesembryanthemum, Sednm, and Senecio furnish excellent ex- 
amples of this type. 

3. The dissected form. The reduction in surface is brought about by 
the division of the leaf blade into narrow linear or thread-like lobes which 

are widely separated. The 
latter are themselves protected 
by a hairy covering or a thick 
cuticle, which is often sup- 
plemented by many rows of 
palisade, or by storage tissue. 
Artemisia, Senecio, and Gilia 
contain species which serve as 
good examples of this type. 

4. The grass form. Xero : 
phytic grasses and sedges have 
narrow filamentous leaves 
with longitudinal furrows 
which serve to protect the 
stomata. The furrows are 
sometimes filled with hairs which are an additional protection, and the 
leaves often protect themselves further by rolling up into a thread-like 
shape. The elongated subulate leaves of J uncus and certain Cyperaceae 
are essentially of this type, although they are usually not furrowed. 

5. The needle form. This is the typical leaf of conifers, in which a 
sweeping reduction of the leaf surface is an absolute necessity. The rela- 
tively small water loss of the needle leaf is still further decreased by a 
thick cuticle, and usually also by hypodermal layers of sclerenchyma. 

6. The roll form. Roll leaves are frequently small and linear. Their 
characteristic feature is produced by the rolling in of the margin on the 
under side, by which an almost completely closed chamber is formed for 
the protection of the stomata which are regularly confined to the lower 
surface of the leaf. - The upper epidermis is heavily cutinized and the lower 




Fig. 34. Diplophyll of Mertensia linearis, 
showing water cells (chresard, 3-9$, light, 1). 
X130. 



HYDROHARMOSE 1 25 

one often protected by hairs. This type is found especially among the 
genera of the Eric ales, but it also occurs in a large number of related 
families. 

7. The scale form. Reduction of leaf surface for preventing excessive 
water loss reaches its logical culmination in the scale leaf characteristic of 
many trees and shrubs, e. g., Cupressus, Tamarix, etc. Scale leaves are 
leathery in texture, short and broad, and closely appressed to the stem, as 
well as often overlapping. 

169. Types of stem xerophytes. In these types the leaves are deciduous 
early in the growing period, reduced to functionless scales, or entirely absent. 
The functions of the leaf have been assumed by the stem, which exhibits 
many of the structural adaptations of the former. Warming 1 has distin- 
guished the following groups : 

1. The phyllode form. The petiole is broadened and takes the place of 
the leaf blade which is lacking. In other cases, the stem is flattened or 
winged, and it replaces the entire leaf. This type occurs in Acacia, 
Baccharis, Genista, etc. 

2. The virgate form. The leaves either fall off early or they are reduced 
to functionless scales. The stems are thin, erect, and rod-like, and are often 
greatly branched. They are heavily cutinized and palisaded, and the stomata 
are frequently in longitudinal furrows. This type is characteristic of the 
Genisteae; it is also found in Ephedra, many species of Polygonum, 
Lygodesmia, etc. 

3. The rush form. In Heleocharis, many species of Juncus, Scirpus, and 
other Cyperaceae, the stem, which is nearly or completely leafless, is cylin- 
drical and unbranched. It usually possesses also a thick cuticle, and several 
rows of dense palisade tissue. 

4. The cladophyll form. In Asparagus the leaves are reduced to mere 
functionless scales, and their function is assumed by the small needle-shaped 
branches. 

5. The flattened form. As in the preceding type, the place of the scale- 
like leaves is taken by cladophylls, which are more or less flattened and leaf- 
like. Riiscus is a familiar illustration of this form. 

6. The thorn form. This is typical of many spiny desert shrubs, in which 
the leaves are lost very early, or, when present, are mere functionless scales. 
The stems have an extremely thick cuticle, and the stomata are deeply 
sunken, as a rule. Colletia and Holacantha are good examples of the type. 

1 Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902. 



126 



THE PLANT 



7. The succulent form. Plants with succulent stems such as the Cac- 
taceae, Stapelia, and Euphorbia have not only decreased water loss by ex- 
treme reduction or loss of the leaves, and the reduction of stem surface, but 
they also offset transpiration by means of storage tissues containing a mu- 
cilaginous sap. The cuticle is usually highly developed and the stomata 
sunken. Thorns and spines are also more or less characteristic features. 




170 Bog plants. Many of the xerophytic types just described are found 
in ponds, bogs, and swamps, where the water supply is excessive, and hydro- 
phytes would be expected. The explanation 
that "swamp xerophytes" are due to the 
presence of humic acids which inhibit absorp- 
tion and aeration in the roots has been 
generally accepted. As Schimper has ex- 
pressed it, bogs and swamps are "physiologi- 
cally dry", i. e., the available water is small 
in amount, in spite of the great total water- 
content. Burgerstein (/. c, 142) has shown, 
however, that maize plants transpire, i. e., 
absorb, three times as much water in a solu- 
tion of 0.5 per cent of oxalic acid as they do 
in distilled water, and that branches of Taxus 
in a solution containing 1 per cent of tartaric 
acid absorb more than twice as much as in 
distilled water. Consequently, it seems im- 
probable that small quantities of humic acids 
should decrease absorption to the extent 
necessary for the production of xerophytes in 
ponds and bogs. Indeed, in many ponds and 
streams, where Heleocharis, Scirpus, Juncus, 
etc., grow, not a trace of acid is discoverable. 
Furthermore, plants with a characteristic 




Fig. 35. Polygonum bistor- 
toides, a stable type: 1, meso- 
phyll (chresard, 25$); 2, xerophyll 
(chresard, 3-5#). X 130. 



hydrophytic structure throughout, such as 



Ranunculus, Caltha, Ludzvigia, Sagittaria, 
etc., are regularly found growing alongside of apparent xerophytes. Many 
of the latter, furthermore, show a striking contrast in size and vigor of 
growth in places w r here they grow both upon dry gravel banks and in the 
water, indicating that the available w T ater-content is much greater in the latter. 
Finally, many so-called "swamp xerophytes" possess typically hydrophytic 
structures, such as air-passages, diaphragms, etc. In spite of a growing 
feeling that the xerophytic features of certain amphibious plants can not 
be ascribed to a low chresard in ponds and swamps, a satisfactory explana- 



HYDROHARMOSE 



127 



tion of them has been found but recently. This explanation has come from 
the work of E. S. Clements already cited, in which it was found that certain 
sun plants underwent no material structural change when grown in the 
shade, and that the same was true also of a few species which grew in two 
or more habitats of very different water-content In accordance with this, 
it is felt that the xerophytic features found in amphibious plants are due to 
the persistence of stable structures, which were developed when these 
species were growing in xerophytic situations. When it is called to mind 
that monocotyledons, and especially the grasses, sedges, and rushes, are 
peculiarly stable, it may be readily understood how certain ancestral 
characters have persisted in spite of a striking change of habitat. Such a 
hypothesis can only be confirmed by the methods of experimental evolution, 
and a critical study of this sort is now under way. 



171. Hydrophytic types. Hydrophytes permit a fairly sharp division 
into three groups, based primarily upon the relation of the leaf surface to 
the two media, air and water. In submerged plants, the leaves are con- 




Fig. 36. Hippuris vulgaris: 1, submerged leaf; 2, aerial leaf. X 130. 



,12 8 



THE PLANT 



stantly below the water ; in amphibious ones, they grow normally in the air. 
Floating plants have leaves in which the upper surface is in contact with the 
air, and the lower in contact with the water. Transpiration is at a maximum 
in the amphibious plant ; it is reduced by half in the floating type, and is 
altogether absent in submerged plants. Aeration reaches a high develop- 
ment in amphibious and floating forms, but air-passages are normally absent 
from submerged forms except as vestiges. Photosynthesis is marked in the 
former, but considerably weakened in the latter. The vascular system, 
which attains a moderate development in the amphibious type, is considerably 
reduced in floating forms, and it is little more than vestigiate in 

submerged ones. 

i. The amphibious type. 
Plants of this type grow in wet 
soil or in shallow water. The 
leaves, are usually large and en- 
tire, the stem well developed, and 
the roots numerous and spread- 
ing. In the majority of cases the 
leaves are constantly above the 
water, but in some species the 
lower leaves are often covered, 
normally, or by a rise in level, 
and they take the form or struc- 
ture of submerged leaves. This 
is illustrated by Callitriche autum- 
nalis, Hippuris vulgaris, Ranun- 
culus delphinif alius, Proserpinaca 
paluslris, Roripa americana, etc. 
The epidermis has a thin cuticle, 
or none at all, and is destitute of 
hairs. The stomata are numer- 
ous and usually more abundant 
The palisade tissue is represented 




Fig. 37. Floating leaf of Spar^anium angus- 
tifolium. X 130. 



on the upper than on the lower surface 
by one or more well-developed rows, but this portion of the leaf is regularly 
thinner than that of the sponge part. The latter contains large air-pas- 
sages, or, in the majority of cases, numerous air-chambers, usually provided 
with diaphragms. The stems are often palisaded, and are characterized by 
longitudinal air-chambers crossed by frequent diaphragms, which extend 
downward through the roots. 

2. The floating type. With respect to form and the structure of the upper 
part of the leaf, floating leaves are essentially similar to those of amphibious 
plants. They are usually lacquered or coated with wax to prevent the 



PHOTOHARMOSE 1 29 

stoppage of the stomata by water. Stomata, except as vestiges, are found 
only on the upper surface, and the palisade tissue is much less developed 
than the sponge, which is uniformly characterized by large air-chambers. 
The stems are elongated, the aerating system is enormously developed, and 
the supportive tissues are reduced. In the Lemnaceae, the leaf and the 
stem are represented by a mere frond or thallus, and the roots are in the 
process of disappearance, e. g., Spirodcla has several, Lemna one, and 
Wo'lffia none. 

3. The submerged type. Both stem and root have been greatly reduced 
in submerged plants, owing to the generalization of absorption and the 
density of the water. The leaves are greatly reduced in size and thickness, 
chiefly, it would seem, for the purpose of insuring readier aeration and great 
illumination. The leaf rnay be ribbon-like, linear, cylindrical, or finely 
dissected. Stomata are sometimes present, but they are functionless and 
vestigial. A distinction into palisade and sponge tissues, when present, 
must also be regarded as a vestige ; the chlorenchym is essentially that of a 
shade leaf. The air chambers are much reduced, and sometimes lacking; 
they function doubtless as reservoirs for air obtained from the water. 

PHOTOHARMOSE 
ADJUSTMENT 

172. Light as a stimulus. In nature, light stimuli are determined by 
intensity and not by quality. A single exception is afforded by those aquatic 
habitats where the depth of water is great, and in consequence of which 
certain rays disappear by absorption more quickly than others. In forests 
and thickets, where the leaves transmit only the green and yellow rays, it 
would appear that the light which reaches the herbaceous layers is deficient 
in red and violet rays. The amount of light transmitted by an ordinary 
sun leaf is so small, however, that it has no appreciable effect upon the 
quality of the light beneath the facies, which is diffuse white light that has 
passed between the leaves. Indeed, it is only in the densest forests that 
distinct sunflecks do not appear. Coniferous forests, with a light value 
less than .005, which suffices only for mosses, lichens, and a few flowering 
plants, show frequent sunflecks. This is convincing evidence that the light 
of such habitats is normal in quality. It warrants the conclusion that in 
all habitats with an intensity capable of supporting vascular plants the light, 
no matter how diffuse, is white light. The direction of the light ray is of 
slight importance in the field, apart from the difference in intensity which 
may result from it. In habitats with diffuse light, the latter comes normally 
and constantly from above. Likewise, in sunny situations, direction can 



I30 THE PLANT 

have little influence, since both the direction and the angle of the incident 
rays change continually throughout the day, and the position of the leaf 
itself is more or less constantly changed by the wind. The influence of 
duration upon the character of light stimuli is difficult to determine. There 
can be no question that the time during which a stimulus acts has a pro- 
found bearing upon the response that is made to it. In nature the problem 
is complicated by the fact that light stimuli are both continuous and 
periodic. The duration- of sunlight is determined by the periodic return of 
night as well as by the irregular occurrence of clouds. Since one is a 
regular, and the other at least a normal happening, it is necessary to con- 
sider duration only .with respect to the time of actual sunlight on sunny 
days, except in the case of formations belonging to regions widely different 
in the amount of normal sunshine, i. e., the number of cloudy days. In 
consequence, duration is really a question of the intensities which succeed 
each other during the day. The differences between these have already been 
shown to fall within the efficient difference for light, and for this reason 
the ratio between the light intensity of a meadow and of a forest is essen- 
tially the ratio between the sums of light intensity for the two habitats, i. e., 
the duration. The latter is of importance only where there is a daily alterna- 
tion between sunshine and shadow, as at the edge of forest and thicket, in 
open woodland, etc. In such places duration determines the actual stimulus 
by virtue of the sum of preponderant intensities. The periodicity of day- 
light is a stimulus to the guard cells of stomata, but its relation to intensity 
in this connection is not clear. 

The amount of change in light intensity necessary to constitute an 
efficient stimulus seems to depend upon the existing intensity as well as 
upon the plant concerned. Apparently, a certain relative decrease is more 
efficient for sun plants than for shade plants. At least, many species sooner 
or later reach a point where a difference larger than that which has been 
efficient no longer produces a structural response. This has been observed 
by E. S. Clements (/. c.) in a number of shade ecads. For example, a 
form of Galium borcale, which grew with difficulty in a light value of .002, 
showed essentially the leaf structure of the form growing in light of .03, 
while the form in full sunlight showed a striking difference in the leaf struc- 
ture. In considering the light stimuli of habitats, it is unnecessary to 
discuss the stimulus of total darkness upon chlorophyllous plants, although 
this is of great importance in experimental evolution and in control experi- 
ment. The normal extremes of light intensity, i. e., those within which 
chlorenchym can function, are full sunshine represented by 1, and a diffuse- 
ness of .002, though small flowering plants have once or twice been found 
in an intensity of .001. The maximum light value, even on high mountains, 
never exceeds 1 by more than an inconsiderable amount, except for the 



PHOTOHARMOSE 131 

temporary concentration due to drops of dew, rain, etc. It seems improba- 
ble that the concentrating effect of epidermal papillae can do much more 
than compensate for the reflection and absorption of the epidermis. Ex- 
perimental study has shown that the maximum intensity in nature may be 
increased several, if not many times, without injurious results and without 
an appreciable increase in the photosynthetic response, thus indicat- 
ing that the efficient difference increases toward the maximum as well as 
toward the minimum. 

173. The reception of light stimuli. Rays of light are received by the 
epidermis, by which they are more or less modified. Part of the light is 
reflected by the outer wall or by the cuticle, particularly when these present 
a shining surface. Hairs diffract the light rays, and hairy coverings con- 
sequently have a profound influence in determining stimuli. The walls 
and contents of epidermal cells furthermore absorb some of the light, 
especially when the cell sap is colored. In consequence of these effects, the 
amount of light that reaches the chlorenchym is always less than that inci- 
dent upon the leaf, and in many plants, the difference is very great. 
According to Haberlandt 1 , the epidermal cells of some shade plants show 
modifications designed to concentrate the light rays. Of such devices, he 
distinguishes two types : one in which the outer epidermal wall is arched, 
another in which the inner wall is deeply concave. Although there can be 
no question of the effect of lens-shaped epidermal cells, their occurrence 
does not altogether support Haberlandt's view. Arched and papillate 
epidermal cells are found in sun plants where they are unnecessary for in- 
creasing illumination, to say the least. A large number of shade plants 
show cells of this character, but in many the outer wall is practically a 
plane. Shade forms of a species usually have the outer wall more arched 
or papillate, but this is not always true, and, in a few cases, it is the lower 
epidermis alone that shows this feature. Finally, a localization of this 
function in certain two-celled papillae, such as Haberlandt indicates for 
Fittonia verschaffclli, does not appear to be plausible. 

The epidermis merely receives the light; the perception of the stimulus 
normally occurs in those cells that contain chloroplasts. The cytoplasm of 
the epidermal ceils, as well as that of the chlorenchym cells, is sensitive to 
light, but the response produced by the latter is hardly discernible in the 
absence of plastids, except in those plants which possess streaming pro- 
toplasm. The daily opening and closing of the stomata, which is due to 
light, is evidently connected with the presence of chloroplasts in the guard 
cells. Naturally, the perception of light and the corresponding response 
occur in the epidermis of many shade and submerged plants which have 

1 Physiologische Pflanzenanatomie. 3d ed., 537. 1904. 



132 THE PLANT 

chloroplasts in the epidermal cells. Such cases merely serve to confirm the 
view that the perception of light stimuli is localized in the chloroplast. In 
conformity with this view, the initial response to such stimuli must be sought 
in the chloroplast, and the explanation of all adaptations due to light must 
be found in the adjustment shown by the chloroplasts. 

174. Response of the chloroplast. The fundamental response of a 
plastid to light is the manufacture of chlorophyll. In the presence of carbon 
dioxide and water, leucoplasts invariably make chlorophyll, and chloroplasts 
replace that lost by decomposition, in response to the stimulus exerted by 
light. The latter is normally the efficient factor, since water is always 
present in the living plant, and carbon dioxide absent only locally at most. 
Sun plants which possess a distinct cuticle, however, produce leucoplasts, 
not chloroplasts, in the epidermal cells, although these are as strongly 
illuminated as the guard cells, which contain numerous chloroplasts. This 
is evidently explained by the lack of carbon dioxide in the epidermis. This 
gas is practically unable to penetrate the compact cuticle, at least in the 
small quantity present in the air. The supply obtained through the 
stomata is first levied upon by the guard cells and then by the cells of the 
chlorenchym, with the result that the carbon dioxide is all used before it 
can reach the epidermal cells. This view is also supported by the presence 
of chloroplasts along the sides and lower wall of palisade cells, where 
there is normally a narrow air-passage, and their absence along the upper 
wall when this is closely pressed against the epidermis, as is usually the 
case. Furthermore, the leaves of some mesophytes when grown in the 
sun develop a cuticle and contain leucoplasts. Under glass and in the 
humid air of the greenhouse, the same plants develop epidermal chloroplasts 
but no cuticle. This is in entire harmony with the well-known fact that 
shade plants and submerged plants often possess chloroplasts in the 
epidermis. Although growing in different media, their leaves agree in 
the absence of a cuticle, and consequent absorption of gases through the 
epidermis. The size, shape, number, and position of the chloroplasts are 
largely determined by light, though a number of factors enter in. No 
accurate studies of changes in size and shape have yet been made, though 
casual measurements have indicated that the chloroplasts in the shade form 
of certain species are nearly hemispherical, while those of the sun form are 
plane. In the same plants, the number of chloroplasts is strikingly smaller 
in the shade form, but exact comparisons are yet to be made. The position 
and movement of chloroplasts have been the subject of repeated study, but 
the factors which control them are still to be conclusively indicated. Light 
is clearly the principal cause, although there are many cases where a marked 
change in the light intensity fails to call forth any readjustment of the 



PHOTOHARMGSE 1 33 

plastids. The position of air-spaces as reservoirs of carbon dioxide and the 
movement of crude and elaborated materials from cell to cell frequently 
have much to do with this problem. Finally, it must be constantly kept in 
mind that the chloroplasts lie in the cytoplasm, which is in constant contact 
with a cell wall. Hence, any force that affects the shape of the cell will 
have a corresponding influence upon the position of the chloroplasts. 
When it is considered that in many leaves these four factors play some part 
in determining the arrangement of the plastids, it is not difficult to under- 
stand that anomalies frequently appear. 

It may be laid down as a general principle that chloroplasts tend to place 
themselves at right angles to rays of diffuse light and parallel to rays of 
sunlight. This statement is borne out by an examination of the leaves of 
typical sun and shade species, or of sun and shade forms of the same 
species. Cells which receive diffuse light, i. e., sponge cells, normally have 
their rows of plastids parallel with the leaf surface, while those in full 
sunlight place the rows at right angles to the surface. This disposition at 
once suggests the generally accepted view that chloroplasts in diffuse light 
are placed in such a way as to receive all the light possible, while those in 
sunlight are so arranged as to be protected from the intense illumination. 
Many facts support this statement with respect to shade leaves, but the need 
of protection in the sun leaf is not clearly indicated. The regular occurrence 
of normal chloroplasts in the guard cells seems conclusive proof that full 
sunlight is not injurious to them. Although the upper wall of the outer 
row of palisade cells is usually free from chloroplasts, yet it is not at all un- 
common to find it covered by them. These two conditions are often found 
in cells side by side, indicating that the difference is due to the presence 
of carbon dioxide and not to light. In certain species of monocotyledons, 
the arrangement of the chloroplasts is the same in both halves of the leaf, 
and there is no difference between the sun and shade leaves of the same 
species. The experimental results obtained with concentrated sunlight, 
though otherwise conflicting, seem to show conclusively that full sunlight 
does not injure the chloroplasts of sun plants, and that the position of 
plastids in palisade cells is not for the purpose of protection. This arrange- 
ment, which is known as apostrophe, is furthermore often found in shade 
forms of heliophytes. In typical shade species, and in submerged plants, 
the disposition of plastids on the wall parallel with the leaf surface, viz., 
epistrophe, is more regular, but even here there are numerous exceptions 
to the rule. 

The absorption of the light stimulus by the green plastid results, under 
normal conditions, in the immediate production of carbohydrates, which in 
the vast majority of cases soon become visible as grains of starch. The 
appearance of starch in the chloroplasts of flowering plants is such a 



134 



THE PLANT 



regular response to the action of light that it is regarded as the normal in- 
dication of photosynthetic activity. The mere presence of chlorophyll is 
not an indication of the latter, since chlorophyll sometimes persists in light 
too diffuse for photosynthesis. The amount of starch formed is directly 
connected with the light intensity, and in consequence it affords a basis for 
the quantitative estimation of the response to light. Two responses to 

light stimuli have a 
direct effect upon the 
amount of transpiration. 
Of the light energy 
absorbed by the chloro- 
plast, only 2.5 per cent 
is used in photosyn- 
thesis, while 95-98 per 
cent is converted into 
heat, and brings about 
marked increase in trans- 
piration. Furthermore, 
in normal turgid plants, 
the direct action of light, 
as is well known, opens 
the stomata in the morn- 
ing and closes them at 
night. 

175. Aeration and 
translocation. The 

movements of gases and 
of solutions through the 
tissues of the leaf are 
intimately connected 
with photosynthesis, and 
hence with responses to 
light stimuli. Aeration 
depends primarily upon 
the periodic opening of 
the stomata, for. while the carbon dioxide and oxygen of the air are able to 
pass through epidermal walls not highly cutinized, the amount obtainable 
in this manner is altogether inadequate, if not negligible. The development 
of sponge tissue or aerenchym is intimately connected with' the stomata. 
The position and amount of aerenchym and the relative extent of sponge 
cells and air-spaces are in part determined by the number and position of the 




Fig. 38. Ecads of Allionia linearis, showing position 
of chloroplasts. The palisade shows apostrophe, the 
sponge epistrophe: 1, sun leaf (chresard, 2-5$, light, 1); 
2, shade leaf (chresard, 11$; light, .012); 3, shade leaf 
(chresard, \\%\ light, .003). X 250. 



PHOTOHARMOSE 



135 



breathing pores. The disposition of air spaces has much to do with the 
arrangement of chloroplasts in both palisade and sponge tissues. Starch 
formation is also dependent upon the presence of air spaces, but, con- 
trary to what would be expected, it seems to be independent of their size, 
since sun leaves, which assimilate much more actively than shade leaves, 
have the smallest air spaces. From this fact, it appears that the rapidity 
of aeration depends very largely upon the rapidity with which the gases are 
used. Translocation likewise affects the arrangement of the chloroplasts 
and the formation of starch. According to Haberlandt, it also plays the 
principal part in determining the form 
and arrangement of the palisade cells. 
Chloroplasts are regularly absent at those 
points of contact where the transfer of 
materials is made from cell to cell, though 
this is not invariably true. Since air pas- 
sages are necessarily absent where cell 
walls touch, it is possible that this disposi- 
tion of the plastids is likewise due to the 
lack of aeration. Translocation is directly 
connected with the appearance of starch. 
As long as all the sugar made by the 
chloroplasts is transferred, no starch ap- 
pears, but when assimilation begins to 
exceed translocation, the increasing con- 
centration of the sugar solution results in 
the production of starch grains. The 
latter is normally the case in all flowering 
plants, with the exception of those that 
form sugar or oil, but no starch. The 
constant action of translocation is practi- 
cally indispensable to starch formation, 
since an over-accumulation of carbohy- 
drates decreases assimilation, and finally 
inhibits it altogether. In consequence, 
translocation occurs throughout the day and night, and by this means the 
accumulated carbohydrates of one day are largely or entirely removed 
before the next. 





Fig. 39. Position of chloroplasts 
in aerial leaf (1) and submerged leaf 
(2 j of Callitriche bifida. X250. 



176. The measurement of responses to light. Responses, such as the 
periodic opening and closing of stomata, which are practically the same for 
all leaves, are naturally not susceptible of measurement. This is also true 
of the transpiration produced by light, but the difficulty in this case is due 



1 



136 THE PLANT 

to the impossibility of distinguishing between the water loss due to light 
and that caused by humidity and other factors. If it were possible to de- 
termine the amount of chlorophyll or glucose produced, these could be used 
as satisfactory measures of response. As it is, they can only be determined 
approximately by counting the chloroplasts or starch grains. The arrange- 
ment of the chloroplasts can not furnish the measure sought, since it does 
not lend itself to quantitative methods, and since the relation to light in- 
tensity is too inconstant. Hesselmann (/. c, 400) has determined the 
amount of carbon dioxide respired, by means of a eudiometer, and has based 
comparisons of sun and shade plants upon the results. As he points out, 
however, light has no direct connection with respiration. Although the 
latter increases necessarily with increased nutrition, the relation between 
them is so obscure, and so far from exact, that the amount of respiration 
can in no wise serve as a measure of the response to light. As a result of 
the foregoing, it is clear that no functional response is able to furnish a 
satisfactory measure of adjustment to light, though one or two have per- 
haps sufficient value to warrant their use. Indeed, structural adaptations 
offer a much better basis for the quantitative determination of the effects 
of light stimuli, as will be shown later. 

In attempting to use the number of chloroplasts or starch grains as a 
measure of response, the study should be confined to the sun and shade 
forms of the same species, or, in some cases, to the forms of closely related 
species. The margin of error is so great and the connection with light 
sufficiently remote that comparisons between unrelated forms or species 
are almost wholly without value. It has already been stated that starch is 
merely the surplus carbohydrate not removed by translocation ; the amount 
of starch, even if accurately determined, can furnish no real clue to the 
amount of glucose manufactured. In like manner, the number of chloro- 
plasts can furnish little more than an approximation of the amount of 
chlorophyll, unless size and color are taken into account. In sun and shade 
ecads of the same species, the general functional relations are essentially 
the same, and whatever differences appear may properly be ascribed to 
different light intensities for the two habitats. The actual counting of 
chloroplasts and starch grains is a simple task. Pieces of the leaves of the 
two or more forms to be compared are killed "and imbedded in paraffin in 
the usual way. To save time, the staining is done in toto. Methyl green is 
used for the chloroplasts and a strong solution of iodine for the starch 
grains. When counts are to be made of both, the leaves are first treated 
with iodin and then stained w T ith the methyl green. The thickness of the 
microtome sections should be less than that of the palisade cells in order 
that the chloroplasts may appear in profile, thus facilitating the counting. 
The count is made for a segment 100 /x in width across the entire leaf. 



PHOTOHARMOSE 137 

Two segments in different parts of the section are counted, and the result 
multiplied by five to give the number for a segment I millimeter in width. 
Although sun and shade leaves regularly differ in size and thickness, no 
correction is necessary for these. Size and thickness stand in reciprocal 
relation to each other in ecads, and thickness is largely an expression of 
the absorption of light, and hence of its intensity. In the gravel, forest, and 
thicket ecads of Galium boreale, counts of the chloroplasts gave the follow- 
ing results. The gravel form (light i) showed 3,500 plastids in the i-mm. 
segment, the forest form (light .03) possessed 1,350, and the thicket form 
(light .002), 1,000. In these no attention was paid to the size and form 
of the plastids in the different leaves, since the differences were inappre- 
ciable. When this is not the case, both factors should be taken into account. 
Starch grains are counted in exactly the same way. Indeed, if care is taken 
to collect leaves of forms to be compared, at approximately the same time 
on sunshiny days, a count of the chloroplasts is equivalent to a count of the 
starch grains in the vast majority of cases. Measurements of the size of 
starch grains can be made with accuracy only when the leaves are killed in 
the field at the same time, preferably in the afternoon. Counts of chloro- 
plasts alone can be used as measures of response in plants that produce 
sugar or oil, while either chloroplasts or starch grains or both may be 
made, the basis in starch-forming leaves. 

Hesselmann (/. c, 379) has employed Sachs's iodine test as a measure 
of photosynthesis. This has the advantage of permitting macroscopic ex- 
amination, but the comparison of the stained leaves can give only a very 
general idea of the relative photosynthetic activity of two or more ecads. 
The iodine test is made as follows '} fresh leaves are placed for a few 
minutes in boiling water, and then in 95 per cent alcohol for 2-5 minutes, 
in order to remove the chlorophyll and other soluble substances. The leaves 
are placed in the iodine solution for ^2-3 hours, or until no further change 
in color takes place. The strength of the solution is not clearly indicated 
by Sachs, who says : "I used an alcoholic solution of iodin which is best 
made by dissolving a large quantity of iodin in strong alcohol and adding 
to this sufficient distilled water to give the liquid the color of dark beer." 
This solution may be approximated by dissolving 1/3 gram of iodin in 100 
grams of 30 per cent alcohol. The stained leaves are put in a white porce- 
lain dish filled with distilled water, and the dish placed in the strong 
diffuse light of a window. The colored leaf stands out sharply against the 
porcelain, and the degree of coloration, and hence of starch content, is 
determined by the following table : 

1 Sachs, J. Em Beitrag zur Kenntniss der Ernahrungsthatigkeit der Blatter. 
Gesammelte Abhandlungen iiber Pflanzenphysiologie. 1:355. 1892. 



138 THE PLANT 

1. bright yellow or leather yellow (no starch in the chlorenchym) 

2. blackish (very little starch in the chlorenchym) 

3. dull black (starch abundant) 

4. coal black (starch very abundant) 

5. black, with metallic luster (maximum starch-content) 

ADAPTATION 

177. Influence of chloroplasts upon form and structure. The begin 
ning of all modifications produced by light stimuli must be sought in the 
chloroplast as the sensitized unit of the protoplasm. Hence, it seems a 
truism to say that the number and arrangement of the chloroplasts de- 
termine the form of the cell, the tissue, and the leaf, although it has not 
yet been possible to demonstrate this connection conclusively by means of 
experiment. In spite of the lack of experimental proof, this principle is 
by far the best guide through the subject of adaptations to light, and in 
the discussion that follows, it is the fundamental hypothesis upon which all 
others rest. The three propositions upon which this main hypothesis is 
grounded are : ( 1 ) that the number of chloroplasts increases with the in- 
tensity of the light; (2) that in shaded habitats chloroplasts arrange them- 
selves so as to increase the surface for receiving light; (3) that chloroplasts 
in sunny habitats place themselves in such fashion as to decrease the surface, 
and consequently the transpiration due to light. In these, there can be 
little doubt concerning the facts of number and arrangement, since they 
have been repeatedly verified. The purpose of epistrophe and apostrophe, 
however, can not yet be stated with complete certainty. 

The stimulus of sunlight and of diffuse light is the same in one respect, 
namely, the chloroplasts respond by arranging themselves in rows or lines 
on the cell wall. The direct consequence of this is to polarize the cell, and 
its form changes from globoid to oblong. This effect is felt more or less 
equally by both palisade and sponge cells, but the disturbing influence of 
aeration has caused the polarity of the cells to be much less conspicuous in 
the sponge than in the palisade tissue. While the cells of both are typically 
polarized, however, they assume very different positions with reference to 
incident light. This position is directly dependent upon the arrangement of 
the plastids as determined by the light intensity. In consequence, palisade 
cells stand at right angles to the surface and parallel with the impinging 
rays ; the sponge cells, conversely, are parallel with the epidermis and at 
right angles to the light ray. Some plants, especially monocotyledons, 
exhibit little or no polarity in the chlorenchym. As a result the leaf does 
not show a differentiation into sponge and palisade, and the leaves of sun 
and shade ecads are essentially alike in form and structure. The form of 



PHOTOHARMOSE 1 39 

the leaf is largely determined by the chloroplasts acting through the cells 
that contain them. A preponderance of sponge tissue produces an extension 
of leaf in the direction determined by the arrangement of the plastids and 
the shape of the sponge cells, viz., at right angles to the light. Shade leaves 
are in consequence broader and thinner, and sometimes larger, than sun 
leaves of the same species. A preponderance of palisade likewise results 
in the extension of the leaf in the line of the plastids and the palisade cells, 
i. e., in a direction parallel with the incident ray. In accordance, sun leaves 
are thicker, narrower, and often smaller than shade leaves. 

178. Form of leaves and stems. In outline, shade leaves are more 
nearly entire than sun leaves. This statement is readily verified by the 
comparison of sun and shade ecads, though the rule is by no means without 
exceptions. In the leaf prints shown in figures 14 and 15, the modification 
of form is well shown in Bursa and Thalictrum; in Capnoides the change 
is less evident, while in Achilleia and Machaer anther a lobing is more pro- 
nounced in the shade form, a fact which is, however, readily explained when 
other factors are taken into account. The leaf prints cited serve as moie 
satisfactory examples of the increase of size in consequence of an increase 
in the surface of the shade leaf, although the leaves printed were selected 
solely with reference to thickness and size or outline. In all comparisons 
of this kind, however, the relative size and vigor of the two plants must be 
taken into account. This precaution is likewise necessary in the case of 
thickness, which should always be considered in connection with amount of 
surface. The relation between surface and thickness is shown by the follow- 
ing species, in all of which the size of the leaf is greater in the shade than in 
the sun. In Capnoides aureum, the thickness of the shade leaf is J / 2 (6:12) 
that of the sun leaf; in Galium boreale the ratio is 5:12, and in Allionia 
linearis it is 3:12. The ratio in Thalictrum sparsiHorum is 9:12, and in 
Machaer anthera aspera 11:12. The thickness of sun and shade leaves of 
Bursa bursa-pastoris is as 14:12, but this anomaly is readily explained by^ 
the size of the plants ; the shade form is ten times larger than the sun 
form. Certain species, e. g., Erigeron speciosus, Potentilla bipinnatifida, 
etc., show no change in thickness and but little modification in size or out- 
line. They furnish additional evidence of a fundamental principle in 
adaptation, namely that the amount of structural response is profoundly 
affected by the stability of the ancestral type. 

The effect of diffuse light in causing stems to elongate, though known for 
a long time, is still unexplained. The old explanation that the plant stretches 
up to obtain more light seems to be based upon nothing more than the co- 
incidence that the light comes from the direction toward which the stem 
grows. Later researches have shown that the stretching of the stem is due 



140 



THE PLANT 



to the excessive elongation of the parenchyma cells, but the cause of the 
latter is far from apparent. It is generally assumed to be due to a lack of 
the tonic action of sunlight, which brings about a retardation of growth in 

sun plants. The evidence in 
favor of this view is not con- 
clusive, and it seems probable 
at least that the elongation of 
the parenchyma cells takes 
place under conditions which 
favor the mechanical stretch- 
ing of the cell wall, but inhibit 
the proper growth of the wall 
by intussusception. It is 
hardly necessary to state that 
the reduced photosynthetic ac- 
tivity of shade plants favors 
such an explanation. What- 
ever the cause, the advantage 
that results from the elongation 
of the internodes is apparent. 
Leaves interfere less with the 
illumination of those below 
them, and the leaves of the 
branches are carried away 
from the stem in such a way as 
to give the plant the best possi- 
ble exposure for its aggregate 
leaf surface. 

179. Modification of the 
epidermis. The development 
of epidermal chloroplasts in 
diffuse light is the only change 
which is due to the direct effect 
of light. This does not often 
occur in the shade ecads of 
sun species, but chloroplasts 
are regularly present in the 
epidermis of woodland ferns 
and of submerged plants. The 
slight development of hairs in sciophilous plants is an advantage, but it 
must be referred to the factors that determine water loss. The significance 





Fig. 40. Isophotophyll of Allionia linearis, 
showing diphotic ecads: 1, light 1; 2, light .012; 
3, light .003. X 130. 



PHOTOHARMOSE 



141 



of epidermal papillae in increasing the absorption of light by shade plants 
has already been discussed. The questions as to what factor has called forth 
these papillae and what purpose they serve must still be regarded as un- 
settled. The increased size of the epidermal cells, which is a fairly constant 
feature of shade ecads, seems to be for the purpose of increasing transloca- 
tion and transpiration, and to bear no relation to light. The extreme 
development of the cells of the epidermis in Streptopus and Limnorchis, 
which grow at the edge of mountain brooks, has been plausibly explained 
by E. S. Clements as a contrivance to increase water loss. The presence 
of a waxy coating, such as that 
found upon the leaves of Im- 
patiens aurea and I. pallida, is 
clearly to prevent the wetting 
of the leaf and the consequent 
stoppage of the stomata. In 
regard to the latter, different 
observers have noted that the 
number of the stomata is 
greater in sun than in shade 
leaves. This holds generally 
for sun and shade species, but 
it is most clearly indicated by 
different ecads of the same 
species. In Scutellaria brit- 
tonii, the sun form possesses 
100 stomata per square milli- 
meter, but in the shade these 
are reduced to 40 per square 
millimeter; the sun leaf of Al- 
lionia linearis has 180 stomata 
to the square millimeter, the 
shade leaf 90. In the stable 
leaf of Erigeron speciosus, 
however, the number of stomata is the same, 180 per square millimeter, for 
sunlight and for diffuse light. The presence of the larger number of 
stomata in the plant exposed to greater loss, which at first thought seems 
startling, is readily explained by the more intense photosynthetic activity 
in the sun. Since the absorption of gases is the primary function of the 
stomata, and transpiration merely secondary, it is evident that sun plants 
must have more stomata than shade plants. This is further explained by 
the fact that the small air passages of sun leaves necessitate frequent inlets, 
which are less necessary in shade leaves with their larger air spaces. In 




Fig. 41. Isophotophyll of Helianthns pumilus, 
showing isophotic ecad: 1, sun leaf; 2, shade leaf 
(light .012). X 130. 



142 



THE PLANT 



shade plants, moreover, the decrease in the number is compensated in some 
measure by the ability of the epidermal cells to absorb gases directly 
from the air. 



180. The differentiation of the chlorenchym. The division of the 
chlorenchym into two tissues, sponge and palisade, is the normal conse- 
quence of the unequal illumination of the leaf surfaces. Exceptions to this 
rule occur only in certain monocotyledons, in which the leaf tissue consists 
of sponge-like cells throughout, and in those stable species that retain more 
or less palisade in spite of their change to diffuse light. The difference in 
the illumination of the two surfaces is determined by the position of the 
leaf. Leaves that are erect or nearly so usually have both sides about equally 
illuminated, and they may be termed isophotic. Leaves that stand more 

or less at right angles to the 
stem receive much more light 
upon the upper surface than 
upon the lower, and may ac- 
cordingly be termed diphotic. 
Certain dorsiventral leaves, 
however, absorb practically as 
much light on the lower side 
as upon the upper. This is 
true of sun leaves with a dense 
hairy covering, which screens 
out the greater part of the 
light incident upon the upper 
surface. It occurs also in 
xerophytes which grow in 
light-colored sands and gravels 
that serve to reflect the sun's 
rays upon the lower surface. In deep shade, moreover, there is no essential 
difference in the intensity of the light received by the two surfaces, and 
shade leaves are often isophotic in consequence. From these examples it 
is evident that isophotic and diphotic leaves occur in both sun and shade, 
and that the intensity of the light is secondary to direction, in so far as the 
modification of the leaf is concerned. 

The essential connection of sponge tissue with diffuse light is conclu- 
sively shown by the behavior of shade ecads, but further evidence of great 
value is furnished by diphotic leaves, and those w T ith hairy coverings. The 
sponge tissue, which in the shade leaf is due to the diffuse light of the 
habitat, is produced m the hairy leaf as a consequence of the absorption and 
diffraction of the light by the covering. In ordinary diphotic leaves, the 




Fig. 42. Diphotophylls of Quercus novi- 
mexicana: 1, sun leaf; 2, shade leaf of the same 
tree (light .06). X 130. 



PHOTOHARMOSE 



H3 



absorption of light in the palisade reduces the intensity to such a degree 
that the cells of the lower half of the leaf are in diffuse light, and are in 
consequence modified to form sponge tissue. The sponge tissue of the 
diphotic leaf is just as clearly an adaptation to diffuse light as it is in 
those plants where the whole chlorenchym is in the shade of other plants 
or of a covering of hairs. As is indicated later, all these relations permit of 
ready confirmation by experiment, either by changing the position of the 
leaf or by modifying the intensity or direction of the light. 

The preceding discussion 
makes it fairly clear that 
sponge tissue is developed 
primarily to increase the light- 
absorbing surface. Because 
of its direct connection with 
photosynthesis, the sponge tis- 
sue is the especial organ of 
aeration, also, and since it 
shows a high development of 
air spaces for this purpose, it 
is inevitably concerned in 
transpiration. It seems to be 
partly a coincidence, however, 
that the sponge is found next 
to the lower surface upon 
which the stomata are most 
numerous. This is indicated 
by artificial ecads of Ranun- 
culus sceleratus, in which 
sponge tissue is unusually de- 
veloped, although the stomata 
are much more numerous upon 
the upper surface. Palisade 
tissue is apparently developed 

primarily as a protection against water loss, particularly that due to the 
absorption of light by the chloroplast. The small size of the intercellular 
passages between palisade cells likewise aids in decreasing transpiration. 
The fact that leaves with much palisade tissue transpire twice a much as 
shade leaves is hardly an objection to this view, as Hesselmann 
(/. c, 442) would think. It is readily explained by the intense photosyn- 
thesis of sun plants, which makes necessary an increase, usually a doubling, 
in the number of stomata, in consequence of which the transpiration is 
increased. 




Fig. 43. A plastic species, Mertensia poly- 
phylla, showing the effect of water upon the 
sponge: 1, chresard 25$; 2, chresard 12$. 
X130. 



i 4 4 



THE PLANT 




181. Types of leaves. Isophotic leaves are equally illuminated and 
possess more or less uniform chlorenchym. Diphotic leaves are unequally 
illuminated, and exhibit a differentiation into palisade and sponge tissues. 
They may be distinguished as isophotophylls and diphotophylls respec- 
tively. 1 Isophotic leaves fall into three 
types based upon the intensity of the light. 
The staurophyll, or palisade leaf, is a sun 
type in which the equal illumination is due 
to the upright .position or to the reflection 
from a light soil, and in which the chlor- 
enchym consists wholly of rows of palisade 
cells. The diplophyll is a special form of 
this type in which the intense light does not 
penetrate to the middle of the leaf, thus re- 
sulting in a central sponge tissue, or water- 
storage tissue. The spongophyll, or sponge 
leaf, is regularly a shade type ; the chlor- 
enchym consists of sponge cells alone. For 
the present at least it is also necessary to 
refer to this group those monocotyledons 
which grow in the sun but contain no pali- 
sade tissue. Diphotic leaves always contain 
both palisade and sponge, though the ratio 
between them varies considerably. Diphot- 
sunny mesophytic habitats. They are 




Fig. 44. A stable species, 
Erigeron speciosus: 1, sun leaf; 2, 
shade leaf (light .03). X 130. 



ophylls are characteristic of 
frequent in xerophytic habi- 
tats as well as in woodlands 
where the light is not too 
diffuse. In the case of stable 
species, this type of structure 
sometimes persists in the dif- 
fuse light of coniferous forests. 
Floating leaves, in which the 
light is almost completely cut 
off from the lower surface, are 
also members of this group. 
Submerged leaves, on the other 
hand, are spongophylls. 

182. Heliophytes and sciophytes. The great majority of sun plants 
possess diphotophylls. This type is represented by Pedicularis procera 




Fig. 45. Spongophyll of Gyrostachys strida 
(light 1). X130. 



Elements, E. S. The Relation of Leaf Structure to Physical Factors. 1905. 



EXPERIMENTAL EVOLUTION I45 

(fig. 32). Plants with isophotophylls are found chiefly in xerophytic places, 
though erect leaves of this type occur in most sunny habitats. The 
staurophyll, in which the protection is due to the extreme development of 
palisade tissue, is illustrated by Aliionia linearis (fig. 40) and Bahia dissecta 
(fig. 33). The diplophyll, which is characterized by a central band of 
sponge tissue or storage cells, is found in Mertcnsia linearis (fig. 34). The 
form of the spongophyll that is found in certain monocotyledons is shown 
by Gyrostachys stricta (fig. 45). The spongophyll (fig. 38:3, 39:2) is 
frequent among plants of deep shade, but as the leaf sections of Aliionia 
(figs. 38, 40) and Quercus (fig. 42) show, the diphotophyll is the rule in 
shade ecads. 

Experimental Evolution 

183. Scope. The primary task of experimental evolution is the de- 
tailed study, under measured conditions, of the origin of new forms in 
nature. As a department of botanical research that is as yet unformed, it 
has little concern with the host of hypotheses and theories which rest 
merely upon general observation and conjecture. A few of these constitute 
good working hypotheses or serve to indicate possible points of attack, but 
the vast majority are worthless impedimenta which should be thrown away 
at the start. It is the general practice to speak of evolution as founded upon 
a solid basis of incontestable facts, but a cursory examination of the evidence 
shows that it is drawn, almost without exception, from observaton alone, 
and has in consequence suffered severely from interpretation. With the 
exception of DeVries's work on mutation, sustained and accurate .investiga- 
tion of the evolution of plants has been lacking. As a result, botanical 
research has been built high upon an insecure foundation, nearly every stone 
of which must be carefully tested before it can be left permanently in place. 
In a field so vast and important as evolution, experiment should far outrun 
induction, and deduction should enter only when it can show the way to a 
working hypothesis of real merit. The great value of DeVries's study of 
mutation as an example of the proper experimental study of evolution has 
been seriously reduced by the fact that the "mutation theory" has carried 
induction far beyond the warrant afforded by experiment. The investigator 
who plans to make a serious study by experiment of the origin of new plant 
forms should rest secure in the conviction that the most rapid and certain 
progress can be made only by the accumulation of a large number of un- 
impeachable facts, obtained by the most exact methods of experimental 
study. 

The general application of field experiment to evolution will render the 
current methods of recognizing species quite useless. It will become im- 
perative to establish an experimental test for forms and species, and to 



I46 THE PLANT 

apply this test critically to every "new species." Descriptive botany, as 
practiced at present, will fall into disuse, as scientific standards come to 
prevail, and in its place will appear a real science of taxonomy. In the latter 
the criteria upon which species are based will be obtained solely by 
experiment. 

184. Fundamental lines of inquiry. There are two primary and 
sharply defined fields of research in experimental evolution, namely, adapta- 
tion in consequence of variation (and mutation), and hybridization. The 
latter constitutes a particular field of inquiry, which is not intimately con- 
nected with the problems of evolution in nature. In the study of specific 
adaptation, two questions of profound importance appear. One deals with 
the effects of ancestral fixity or plasticity in determining the amount of 
modification produced by the habitat. These are fundamental problems, 
and a solution of them can not be hoped for until exact and trustworthy data 
have been provided by numerous experimental researches. It thus becomes 
clear that the principal, if not the sole task of experimental evolution for 
years to come is the diligent prosecution of accurate and prolonged experi- 
ment in the modification of plant forms. It seems inevitable that this will 
be carried on along the lines that have already been indicated. Plants will 
be grown in habitats of measured value, or in different intensities of the 
same factor. The relation between stimulus and adjustment will form the 
basis of careful quantitative study, and the final expression of this relation 
in structural modifications will find an exact record in drawings, photo- 
graphs, exsiccati, and biometrical measures. The making of an accurate 
and complete record of the whole course of each experiment of this sort 
is an obligation that rests upon every investigator. Studies in experimental 
evolution will prove time-consuming beyond all other lines of botanical 
research, and the work of one generation should appear in a record so 
perfect that it can be used without doubt or hesitation as a basis for the 
studies of the succeeding generation. 

185. Ancestral form and structure. The significance of the fact that 
some species have been found to remain unaltered structurally under changes 
of habitats that produced striking modifications in others has already been 
commented upon. It is hardly necessary to indicate the important bearing 
which this has upon evolution. The very ability of a plant to undergo 
modification, and hence to give rise to new forms, depends upon the degree 
of fixity of the characters which it has inherited. Stable plants are less 
susceptible of evolution than plastic ones. The latter adapt themselves to 
new habitats with ease, and in each produce a new form, which may serve 



EXPERIMENTAL EVOLUTION I47 

as the starting point of a phylum. There is at present no clue whatever as 
to what calls forth this essential difference in behavior. This is not surpris- 
ing in view of the fact that there have been no comparative experimental 
studies of stable and plastic species. Until these have been made, it is im- 
possible to do more than to formulate a working hypothesis as to the effect 
of stability, and an explanation of the forces which cause or control it is 
altogether out of the question. 

186. Variation and mutation. New forms of plants are known to arise 
by three methods, viz., variation, mutation, adaptation. The evidence in 
support of these is almost wholly observational, and consequently more or 
less inexact, but for each there exist a few accurate experiments which 
are conclusive. Origin by variation and subsequent selection is, the essence 
of the Darwinian theory of the origin of species. According to this 
the appearance of a new form is due to the accumulation, and selection, 
through a long period, of minute differences which prove advantageous to 
the plant in its competition with others in nature, or are desirable under 
cultivation. Slight variations appear indiscriminately in every species. 
Their cause is not known, but since they are found even in the most uniform 
habitats, it is impossible to find any direct connection between them and the 
physical factors. In the case of origin by mutation, the new form appears 
suddenly, with definite characteristics fully developed. Selection, in the 
usual sense of the term, does not enter into mutation at all, though the 
persistence of the new form is still to be determined by competition. Muta- 
tions are known at present for only a few species, and their actual appearance 
has been studied in a very few cases. Like variations, they are indiscrimi- 
nate in character. The chief difference between them is apparently one of 
degree. Indeed, mutation lends itself readily to the hypothesis that it is 
simply the sudden appearance of latent variations which have accumulated 
within the plant. DeVries regards constancy as an essential feature of 
mutation, but the evidence from the mutants of Onagra is not convincing. 
Indeed, while there can be no question of the occurrence of mutation in 
plants, a fact known for many years, the facts so far brought forward in 
support of the "mutation theory" fall far short of proving "the lack of 
significance of individual variability, and the high value of mutability for 
the origin of species." 1 Mutations do not show any direct connection with 
the habitat, but their sudden appearance suggests that they may be latent 
or delayed responses to the ordinary stimuli. Origin by adaptation is the 
immediate consequence of the stimuli exerted by the physical factors of a 

'DeVries, H. Die Mutationstheorie, 1:6. 1901. 



I48 THE PLANT 

habitat. This fact distinguishes it from origin by variation, or by mutation. 
The new form may appear suddenly, often in a single generation, or grad- 
ually, but in either case it is the result of adaptation that is necessarily 
advantageous, because it is the result of adjustment to controlling physical 
factors. Origin by adaptation is perhaps only a special kind of origin by 
variation, but this might be said with equal truth of mutation. New forms 
resulting from adaptation are like those produced from mutation, in that 
they appear suddenly as a rule and without the agency of selection. They 
are essentially different, inasmuch as their cause may be found at once in 
the habitat, and since a reversal of stimuli produces, in many cases at 
least, a reversion in form and structure to the ancestral type. 

A valid distinction between forms or species upon the basis of constancy 
is impracticable at the present time. It is doubtful that such a distinction can 
ever be made in anything like an absolute sense, since all degrees of fluctua- 
tion may be observed between constancy and inconstancy. In all events, 
it is gratuitous to make constancy the essential criterion in the present state 
of our knowledge. So little is certainly known of it that it is equally un- 
scientific to affirm or to deny its value, and even a tentative statement can 
not be ventured until a vast amount of evidence has been obtained from ex- 
periment. Accordingly, there is absolutely no warrant, other than tradition, 
for limiting the term species to a constant group. In the evolutionary sense, 
a species is the aggregate ancestral group and the new forms which have 
sprung from it by variation, mutation, or adaptation. It should not be 
regarded as an isolated unit for purposes of descriptive botany; indeed, 
its use in this connection is purely secondary. It is properly the unit to 
be used in indicating the primary relationships which are the result of 
evolution. 

On the basis of their actual behavior in the production of new forms, 
species may be distinguished as variable, mutable, or adaptable. The new 
form which results from variation is a variant; the product of mutation is 
a mutant, and that of adaptation, an ecad. The following examples serve 
to illustrate these distinctions. Machaer anther a canescens, judging from 
the numerous minute intergrades between its many forms, is a variable 
species, i. e., one in which forms are arising by the gradual selection of 
small variations. It apparently comprises a large number of variants, 
M. canescens aspera, superba, ramosa, viscosa, etc. Onagra lamarckiana 
is a mutable species : it comprises many mutants, e. g., Onagra lamarckiana 
gigas, O. /. nanella, O. I lata, etc. Galium boreale is an adaptable species: 
it possesses one distinct ecad, Galium boreale hylocolum, which is the shade 
form of the species. 



EXPERIMENTAL EVOLUTION I49 

187. Methods. The best of all experiments in evolution are those that 
are constantly being made in nature. Such experiments are readily dis- 
covered and studied in the case of origin by adaptation ; variants present 
much greater difficulties, while mutants are very rare under natural condi- 
tions. The method which makes use of these experiments may be termed 
the method of natural experiment. The number of ecads which appear 
naturally in vegetation is limited, however, and it is consequently very 
desirable to produce them artificially, by the method of habitat culture. 
This method, while involving more labor than the preceding, yields results 
that are equally conclusive, and permits the study of practically every 
species. The method of control culture, which is carried on in the plant- 
house, naturally does not possess the fundamental value of the field methods. 
It is an invaluable aid to the latter, however, since it permits the physical 
factors to be readily modified and controlled. All these methods are based 
on the indispensable use of instruments for the measurement of physical 
factors. 

METHOD OF NATURAL EXPERIMENT 

188. Selection of species. Species that are producing variants or ecads 
are found everywhere in nature; those which give rise to mutants seem, 
however, to be extremely rare. Consequently, mutants can not be counted 
upon for experimental work, and their study scarcely needs to be con- 
sidered. When a mutant is discovered by some fortunate chance, the 
mutable species from which it has sprung, and related species as well, 
should be subjected to the most critical surveillance, in the hope that new 
mutants will occur or the original one reappear. On account of the sud- 
denness with which they appear, mutants do not lend themselves readily to 
natural experiment, and after they have once been discovered, inquiry into 
the causes and course of mutation is practicable only by means of habitat 
and control cultures. Among variable species, those are most promising 
that show a wide range of variation and are found in abundance over 
extensive areas. A species which occurs in widely separated, or more or 
less isolated areas, furnishes especially favorable material for investigation, 
since distance or physical barriers partly eliminate the leveling due to con- 
stant cross-fertilization. The individuals or groups which show appreciable 
departure from the type are marked and observed critically from year to 
year. The direction of the variation and the rapidity with which small 
changes, are accumulated can best be determined by biometrical methods. 
Representative individuals of the species and each of its variants should 
likewise be selected from year to year. After being photographed, these are 
preserved as exsiccati, and with the photographs constitute a complete 



15° THE PLANT 

graphic record of the course of variation. When the latter is made evident 
in structural feature also, histological slides are an invaluable part of the 
record. 

Polydemic- species are by far the best and most frequent of all natural 
experiments. In addition to plants that are strictly polydemic, i. e., grow 
in two or more distinct habitats, there are a large number which occur in 
physically different parts of the same habitat. The recognition of polydemics 
is the simplest of tasks. As a rule, it requires merely a careful examination 
of contiguous formations in order to ascertain the species common to two 
or more of them. The latter are naturally most abundant along the eco- 
tones between the habitats, and, as a result, transition areas and mixed 
formations are almost inexhaustible sources of ecads. Many adaptable 
species are found throughout several formations, however, and such are 
experiments of the greatest possible value. Not infrequently species of the 
manuals are seen to be ecads, in spite of their systematic treatment, and to 
constitute natural experiments that can be readily followed. Finally, it 
must be kept in mind that some polydemics are stable, and do not give rise 
to ecads by structural adaptation. They not only constitute extremely in- 
teresting experiments in themselves, but they should also be very carefully 
followed year by year, since it seems probable that the responses are merely 
latent, and that they will appear suddenly in the form of mutants. In 
natural experiments it is sometimes difficult to distinguish which form is the 
ecad and which the original form of the species. As a rule, however, this 
point can be determined by the relative abundance and the distribution, but 
in cases of serious doubt, it is necessary to appeal to experimental cultures. 

Although habitats differ more or less with respect to all their factors, the 
study of polydemics needs to take into account only the direct factors, 
w r ater-ccntent, humidity, and light. Humidity as a highly variable factor 
plays a secondary part, and in consequence the search for ecads may be 
entirely confined to those habitats that show efficient differences in the 
amount of water-content or of light. Temperature, wind, etc., do not pro- 
duce ecads, and may be ignored, except in so far as they affect the direct 
factors. Complexes of factors, such as altitude, slope, and exposure, are 
likewise effective only through the action of the component simple factors 
upon water and light. The influence of biotic factors is so remote as to 
be negligible, especially in view of the fact that ecads are necessarily 
favorable adaptations, and are in consequence little subject to selective 
agencies. The essential lest of a habitat is the production of a distinguish- 
able ecad, but a knowledge of the water-content and light values of the 
habitats under examination is a material aid, since a minute search of each 
formation is necessary to reveal all the ecads. It is evident that habitats or 



EXPERIMENTAL EVOLUTION 151 

areas that do not show efficient differences of water or light will contain no 
ecads of their common species, and also that extreme differences in the 
amount of either of these two factors will preclude origin by adaptation to 
a large degree, on account of the need for profound readjustment. The 
general rule followed by most polydemics is that sun species will give rise 
to shade forms, and vice versa, and that xerophytes will produce forms of 
hydrophytic tendency, or the converse, when the areas concerned are not too 
remote, and the water or light differences are efficient, but not inhibitive. 
Some species are capable of developing naturally two series of ecads, one 
in response to light, the other to water-content, but they, unfortunately, have 
been found to be rare. Greatly diversified regions, such as the Rocky 
mountains, in which alternation is a peculiarly striking feature of the vegeta- 
tion, are especially favorable to the production of ecads, and hence for the 
study of natural experiments in origin by adaptation. 

189. Determination of factors. For the critical investigation of the 
origin of new forms, an exact knowledge of the factors of the habitat, both 
physical and biotic, is imperative. In the case of variable species, these 
factors determine what variations are of advantage, and thereby the direc- 
tion in which the species can develop. They are the agents of selection. 
With mutants, the factors of the habitat are apparently neither causative 
nor selective, though it seems probable that further study of mutants will 
show an essential connection between mutant and factor. In any event, the 
persistence of a mutant in nature, and its corresponding ability to initiate 
new lines of development, is as much dependent upon the selection exerted 
by physical and biotic factors as is the origin of variants. Physical factors 
are causative agents in the production of ecads, as has been shown at length 
elsewhere. The form and structure of the ecad are the ultimate responses 
to the stimuli of light or water-content, and the quantitative determination 
of the latter is accordingly of the most fundamental importance. The meas- 
urement of factors has been treated so fully in the preceding chapters that 
it is only necessary to point out that the thorough investigation of habitats 
by instruments is as indispensable for the study of experimental evolution as 
for that of the development and structure of the formation. Furthermore, 
it is evident that a knowledge of physical factors is as imperative for habitat 
and control cultures as for the method of natural experiment. In the latter, 
however, the biotic factors demand unusual attention, since pollination, iso- 
lation, etc., are often decisive factors in origin by variation and in the per- 
sistence of mutants. 

Measurements of adjustment, i. e., functional response to the direct 
factor concerned, are extremely valuable, but not altogether indispensable 



152 THE PLANT 

to research in experimental evolution. This is due to the fact that a knowl- 
edge of adjustment is important in tracing the origin of new forms only 
when adjustment is followed by adaptation, and in all such cases the ratio 
between the two processes seems to be more or less constant. In the present 
rudimentary development of the subject, however, it is very desirable to 
make use of all methods of measuring functional responses to water and 
light that are practicable in the field. Certain methods that are difficult of 
application in nature may be used to advantage in control cultures, and the 
results thus secured can be used to interpret those obtained from natural 
experiments and field cultures. 

190. Method of record. As suggested elsewhere, there are four im- 
portant kinds of records, which should be made for natural experiments, 
and likewise for habitat and control cultures. These are exsiccati, photo- 
graphs, biometrical formulae and curves, and histological sections. These 
serve not merely as records of what has taken place, but they also make it 
possible to trace the course of evolution through a long period with an 
accuracy otherwise impossible, and even to foreshadow the changes which 
will occur in the future. The possibility of doing this depends primarily 
upon the completeness of the record, and for this reason the four methods 
indicated should be used conjointly. In the case of ecads and mutants, 
exsiccati, photographs, and sections are the most valuable, and in the ma- 
jority of cases are sufficient, since both ecads and mutants bear a more 
distinctive impress than variants do. On the other hand, since variations 
are more minute, the determination of the mean and extreme of variation 
by biometrical methods is almost a prerequisite to the use of the other three 
methods, which must necessarily be applied to representative individuals. 

Exsiccati and photographs are made in the usual way for plants, but it 
is an advantage to photograph each ancestral form alongside of its proper 
ecads, mutants, or variants, in addition to making detail pictures of each 
form and of the organs which show modification. In the collection of 
material for histological sections, which deal primarily with the leaf or with 
stems in the case of plants with reduced leaves, a few simple precautions 
have been found necessary. Whenever possible, material should be killed 
where it is collected, since in this way the chloroplasts are fixed in their 
normal position. In case leaves that can not be replaced easily have become 
wilted, an immersion of 5-6 hours in water will make it possible to kill them 
without shrinkage. In selecting leaves, g'reat pains must be taken to collect 
only mature leaves. When the plants have a basal rosette, or distinct 
radical leaves, mature leaves are taken from both stem and base. In all cases 
where the two surfaces of the leaf can not be readily distinguished, the 
upper one is clearly marked. 



EXPERIMENTAL EVOLUTION 1 53 

METHOD OF HABITAT CULTURES 

191. Scope and advantages. By means of experiments actually made 
in the field, practically every species that is capable of modification can be 
made to produce new forms, the origin of which can be traced in the manner 
already indicated. Field experiments of this sort are especially favorable to 
the production of ecads from adaptable species. No attempt has yet been 
made to apply it to mutable or variable species, but its ultimate application 
to these does not seem at all impossible. The chief advantage of the method 
of habitat cultures is seen in the great range of choice in selecting the plant 
for experiment, and the habitat or area in which the experiment is carried 
out. A polydemic species which already has one or more ecads can be ex- 
tended to a number of different habitats of known value, and a complete 
series of ecads obtained, based either upon water-content, or light, or upon 
both. On the other hand, an endemic species, or one brought from a remote 
flora, can be placed in as many habitats as desired, and the appearance of 
ecads followed in each. Frequently, results of much value are obtained in a 
diversified habitat by growing its most plastic species in those areas which 
show the greatest differences in water-content or light intensity. Habitat 
cultures give results which are practically as perfect as those obtained from 
natural experiments, since the course of adaptation in no wise depends upon 
whether the agent by which the seed or propagule is carried into the new 
habitat is natural or artificial. Cultures of this kind further possess the 
distinct advantage of permitting more or less modification of the physical 
factors themselves. However, when it is desirable to have the factors under 
as complete control as possible, it is necessary to use the method of control 
cultures in the planthouse. 

192. Methods. All field experiments in evolution are based upon a 
change of habitat. The latter is accomplished by the modification of the 
habitat itself, or by the transfer of the species to one or more different hab- 
itats, or to different areas of the same habitat. In both cases the choice of 
habitats is made upon the basis of efficient differences of water-content or 
light. Saline situations do not constitute an exception, since the chresard 
is really the effective stimulus. Cultures at different altitudes, which afford 
striking results, appear to concern several factors, but in the final analysis, 
water-content and humidity are alone found to be really formative. Cul- 
tures may furthermore be distinguished as simple or reciprocal. Simple 
cultures are those in which a species is transferred to one or more habitats, 
or in which a habitat is modified in one or more ways. Reciprocal cultures 
are possible only with polydemic species, or with endemics after ecads have 



154 THE PLANT 

been produced by experiment. Modification or transfer is made in the usual 
way, but reciprocally, i. e., the original form is transferred to the habitat of 
the ecad, and the latter to the habitat of the former; or the shade in which 
some individuals of the ecad are growing may be destroyed, and at the same 
time individuals of the type may be shaded. Both transfer and modification 
may be applied to the same species, but since the same measured change of 
factor can be obtained in either way, the use of both is undesirable, with the 
exception of the rare cases where they serve as checks upon each other. 
The transfer of a seed or plant is so much simpler and more convenient 
that this method is the one regularly used. It sometimes happens, however, 
that a change of water-content or light intensity is readily and conveniently 
made, and is desirable for other reasons. 

It is evident that both transfer and modification require that the factor 
records of the various habitats or areas be as full as possible, at least so far 
as water -content, humidity, and light are concerned. In the case of the 
areas that are to be modified, these factors are determined before the change 
is made. Afterward they are read from time to time during the growing 
season, and are also checked by readings made near at hand in the unmodi- 
fied formation. The readings made in the beginning should correspond 
closely to the check readings, but in case of disagreement the latter are to 
be taken as conclusive. 

193. Transfer. After the species to be used for experiment has been 
chosen, the various habitats or areas selected, and the direct factors meas- 
ured by instruments, the actual transfer of the individuals is made by means 
of seeds, preferably in autumn, though the results are practically the same 
if seeds are kept over the winter and planted at the opening of spring. The 
natural method is to scatter the seeds in the place selected, as though they 
had been carried by the usual agents of migration. The mortality is usu- 
ally great in such case, however, and the chances of success are increased 
by actually planting the seeds. This is the method which has been used in 
making cultures of species of the European Alps on the summit of Mount 
Garfield in the Rocky mountains. The number of seeds used is recorded in 
order to obtain some estimate of germination and competition. While the 
use of the seed or disseminule possesses the great advantage of making the 
experiment essentially a natural one, the transfer of rosettes, seedlings, or 
young plants makes the results more certain, and consequently saves time, 
even though the actual transfer is somewhat more difficult. It is hardly 
necessary to point out that the removal of the plant should be made with the 
greatest care. The best success is obtained by making the transfer on cloudy 
or rainy days, and when shade plants are to be placed in sunny situations, 
they should be transplanted late in the afternoon. When the task of carry- 



EXPERIMENTAL EVOLUTION 



155 



ing them is not too great, it is a distinct advantage to move a number of 
individuals in the same block of earth. The transfer of mature plants is 
inadvisable, except for those perennials which can not readily be secured in 
an early stage. This naturally does not apply to woody plants, evergreen 
herbs, mosses and lichens ; the last two may be transferred at any time with 
satisfactory results. Each culture is carefully marked with stakes, and 
definitely located by means of landmarks. 




Fig. 46. Series for producing hydrophytic forms under control: 
1, amphibious; 2, floating; 3, competition; 4, submerged. 

Reciprocal transfers may be made by means of seed or plant. Since the 
experiment is a complex one, all the care possible should be taken to make 
sure that the plants become established in the reciprocal situations, and con- 
sequently, it is often advisable to transfer both seeds and plants. Reciprocal 
transfer is of paramount value in solving the problem which bog plants 
present. A slight modification of the method makes it possible to obtain 
experimental evidence of the polyphyletic origin of species in consequence 
of adaptation. In an experiment mentioned elsewhere, the transfer of 
Kuhnistera purpurea to the area occupied by K. Candida, and vice versa, is 
designed to show whether one has been derived from the other. If the two 
species are moved into an area which contains more water than that usually 
occupied by K. purpurea, and less water than is found where K. Candida 
habitually grows, the resulting modifications will throw much light upon the 



156 



THE PLANT 



origin of polyphyletic species. In this connection, it hardly needs to be 
pointed out that this simple transfer of a species to several separated areas 
of a new habitat may often furnish complete proof that a new form may 
arise at different times, and at different places. 



194. Modification of the habitat. Efficient changes in the habitat are 
brought about by increasing or decreasing the water-content, or by varying 
the light intensity between sunshine and the diffuse light of deep forests. 
Humidity can not well be regulated except in so far as it is connected with 
water-content. Since its effects merge with those of the latter, its modifi- 
cation is unnecessary. An increase in water-content is readily brought about 
by irrigation. A stream may be dammed and its water allowed to spread 
over the area to be studied, or the water may be carried to the proper place 

by deflecting the stream or 
by digging a canal. The 
construction of earth reser- 
voirs makes it possible to 
obtain almost any per cent 
of soil water by varying the 
size of the reservoir or the 
height of the wall or bank. 
Near a base station, such as 
Minnehaha, where there is a 
simple system of water- 
works, the experimental area 
may be watered whenever 
desirable by means of a hose. 
Water-content may be read- 
ily decreased by drainage, or 
by the deflection of a stream. 
When such means are not 
available, as in the case of 
extensive marshes, hum- 
mocks may be used or con- 
structed, and the soil blocks 
containing plants placed up- 




Fig. 47. Control ecad of Ranunculus sceleralus, 
holard 10£ (50 cc). 



on them. By the use of sand or gravel, the water-content of mesopEytic 
areas can be reduced in a similar manner, or by surrounding the plant in situ 
with either of these soils which hold little water. In meadows, especially, 
the addition of a large quantity of alkaline salts decreases the amount of 
available water, while the holard may be reduced by denuding the soil about 
the plants concerned. 



EXPERIMENTAL EVOLUTION 



157 



In sunny habitats, the light intensity is most easily reduced by means of 
cloth awnings, which can be put in place conveniently. It is not a difficult 
matter to produce effective shade by using shrubs or small trees for this pur- 
pose. This plan is especially advantageous in habitats too remote to make 
frequent visits feasible. When a shrub or tree is used, the experiment nec- 
essarily requires a longer time, though this disadvantage is partly compen- 
sated by the fact that 
the shelter requires 
practically no attention 
after the shrub is once 
established. Forest 
plantations furnish ex- 
cellent examples of this 
kind of experiment. On 
the other hand, clear- 
ings afford the only ex- 
amples of habitats mod- 
ified in such manner as 
to increase the light. In 
nature, the diffuse light 
in which shade plants 
grow is due to the pres- 
ence of tall plants, 
chiefly shrubs and trees, 
and an increase in the 
light intensity is possi- 
ble only through the 
thinning-out or removal 
of the plant screen. 
This is a task of consid- 
erable magnitude in for- 
ests, but it can be readily accomplished in thickets and at the edges of wood- 
lands. It is quite practicable to establish a series of awnings or clearings of 
various light values, but the labor required is hardly worth while when it is 
recalled that the method of transfer makes it possible to take advantage of 
the various intensities already found in nature. 

METHOD OF CONTROL CULTURES 




Fig. 48. Control ecad of Ranunculus sceleratus, 
holard40# (200 cc). 



195. Scope and procedure. Control experiments are necessarily 
carried on in the planthouse, since factors can be controlled in the field 
only with great difficulty. Their greatest value is in connection with 



I58 THE PLANT 

experiments that are being carried on in the habitat, but they also 
constitute an invaluable means of independent research, since it is not at 
all difficult to approximate the conditions of a habitat, especially with 
reference to water-content and light. The essential feature of the method 
is that the less important factors are equalized as far as possible, while the 
direct factors, water-content and light, are under the complete control of 
the investigator. By the equalization of humidity and temperature is meant 
experimentation in which all the plants of each experiment are subjected to 
the same amounts of these factors. It is a matter of no importance what- 
ever whether the humidity and temperature are constant or variable. In 
the case of soil, which is not a variable, it naturally happens that the plants 
are placed once for all in the same soil mixture. Batteries consisting of 
thermograph and psychrograph have been kept in the different control 
houses, but although used at first to give some idea of the hourly and daily 
fluctuations of temperature and humidity, they have slight bearing upon the 
evolution of new forms under control. For use in connection with supple- 
mentary experiments in adjustment and adaptation, the batteries have 
proved to be indispensable. Control experiments are regularly made in 
series which are planned with reference to as many modifications as the 
efficient difference of the factor and the plasticity of the species con- 
cerned permit. 

196. Water=content series. An account of the experiments which 
have been carried on for four generations with Ranunculus sceleratus will 
serve to show the application of culture methods to the origin of new forms 
in response to varying water-content. This species was chosen because it 
grows readily in the planthouse, is plastic, and, since it is naturally am- 
phibious, permits of much modification in both directions. The smallest 
amount of water per day under which the seedlings would grow was found 
to be 25 cc. This was taken as one extreme for the series, and deep water 
in which the plant could be submerged as the other. An arbitrary series 
was tentatively made as follows: 25 cc, 50 cc, 100 cc, 150 cc, 200 cc, 
mud, shallow water, and deep water. Further study justified these 
divisions, since the first six gave efficient differences in water-content, and 
the resulting forms all showed differences of structure as well as of growth 
and form. Seedlings of the same age, and as nearly alike as possible, were 
transplanted to large pots of which there were four for each of the first 
six ; they were placed in half-barrels for mud and floating forms, and in a 
barrel for submerged forms. After a few days, when they had become well 
established, the plants in the pots were watered in the amounts indicated, as 
often as was necessary to keep the most xerophytic form alive ; the soil for 



EXPERIMENTAL EVOLUTION 



J 59 



the mud form was kept covered with a thin film of water; the leaves of the 
form in shallow water were kept floating on the surface, and those of the 
last form submerged just below the surface. The water in which the sub- 
merged form grew was aerated by means of a spigot near the bottom of the 
barrel. From time to 
time water-content de- 
terminations were made 
of the soil in the pots 
until it was definitely as- 
certained that the holard 
was practically constant. 
The nine new forms ob- 
tained by adaptation 
showed striking differ- 
ences in vigor and 
growth, as may be seen 
from the figures. In all 
cases, these were ac- 
companied by distinct 
and often striking dif- 
ferences in the number 
and position of the sto- 
mata, the amount of 
sponge and palisade tis- 
sues, and the develop- 
ment of air passages. 
Photographs were made 
of a typical plant of 
each form, and the dif- 
ferent leaf structures 




Fig. 49. Floating form of Ranunculus sceleratus 
grown under control. 



were preserved in permanent mounts. The xerophytic and the submerged 
form were unable to produce flowers, and it was necessary to develop them 
anew in each generation. The other forms fruited abundantly, and the 
succeeding generations of each form were produced from plants which had 
grown the year before in the same conditions. In addition to the develop- 
ment of a series of new water-content forms, this experiment was begun 
in the hope of determining whether the modifications of a plastic species 
tend to become fixed if each new form is grown constantly under the same 
conditions. A period of four years is too short, however, to throw much 
light upon this problem. 



l60 THE PLANT 

Helianthus. annuus has been used for other series of experiments, in 
which alkaline salts or different soils are employed to vary the water-content. 
These are more complex and hence are not as satisfactory as the series 
described above, but they are valuable for the light they throw upon the 
behavior of plants in similar conditions in nature. In the case of soil, 
however, the adaptation may be referred to water-content alone, if 
thoroughly leached sands and gravels are used, so that the difference is 
solely one of water-retaining power. 

197. Light series. Cloth tents have been found the most satisfactory 
means of obtaining different light intensities in the planthouse. The cloth 
permits the air to circulate to a considerable degree, and in consequence 
the equalization of humidity and temperature is much more complete than 
in the glass houses first employed. The cloth tents, or shade tents as they 
are called, are cubical, each dimension being i meter. The series which 
has been most used consists of three tents : the first is made of cheese- 
cloth and has a light value of .1 ; the second is of thin muslin, and has a 
value of .04, while the third is made of dark cambric and the light is reduced 
to .01. A more desirable series is one with five tents, which have approxi- 
mately the following light intensities: .1, .05, .01, .007, .003. Plants grown 
in shade tents should be repotted as often as they will permit in order to 
increase the aeration of the soil. The amount of water given them must 
also be decreased as the shade increases. Mesophytic species give the best 
results in shade tents, xerophytes thrive less well, and amphibious plants do 
net grow at all except in the brightest lig-ht. Excellent results have been 
obtained with Helianthus, Taraxacum, Gaum, and Onagra, while Ran- 
unculus sceleratus is unable to produce flowers and seeds in a light intensity 
of .01. 

A number of important supplementary experiments have been made in 
connection with light tents. These do not result in the production of new 
forms, but they throw much light upon it. Plants have been placed in 
the shade tents so that certain leaves would be in the sun and others in 
the shade. Young leaves have been fixed at various angles with the 
stem, and they have been revolved 90 ° or 180 in order to change the rela- 
tion of their surfaces. Soils of different colors, e. g., loam and sand, have 
been used to determine the effect of light reflected from their surfaces. 
Shade tents make it possible to illuminate plants from the top, bottom, or 
side, and to carry on a large number of fundamental experiments in adjust- 
ment and adaptation. 



CHAPTER IV. THE PLANT FORMATION 
Methods of Investigation and Record 

198. The need of exact methods. The use of instruments in the study of 
the habitat has made it evident that the loose methods of descriptive ecology 
were altogether inadequate to the accurate investigation of the formation. 
This feeling has been heightened by the. recognition of the fact that vegeta- 
tion exhibits both development and structure, and is, in consequence, open 
to exact methods of inquiry. In the search for feasible methods, it was 
quickly seen that the quadrat, first 1 used for determining the abundance of 
species, furnished the key to the problem. Accordingly, the principle un- 
derlying it, viz., that of intimate detailed study and record, was developed 
and extended in such a way as to give rise to a number of methods of 
precision. These have been applied in the field for several years with 
signal success, and they are here described in the conviction that they con- 
stitute a satis facto rv svstem, if not, indeed, the only one for the exact study 
of formations. 

There has been a growing appreciation of the fact that the superficial 
methods of descriptive ecology made it impossible to build upon such a 
foundation, and they, indeed, were making actual progress in the field of 
ecology' more and more difficult. Ecologists have now begun to see clearly 
that precise methods are as indispensable in the habitat as they are to the 
study of the structure and modification of the plant. For some reason, 
however, they have been slow to perceive that accuracy in the investigation 
of the cause, the habitat, is a fruitless task unless it be followed by corres- 
ponding exactness in the study of the effect, the formation. After having 
urged the fundamental necessity of instrumental methods, for six or seven 
years, both in season and out of season, the writer does not feel called 
upon to further plead the cause of the quadrat. The final acceptance of the 
instrument was inevitable if progress were to be made in the habitat, and 
it is just as obvious that the quadrat must be accepted if the study of the 
habitat is to bear fruit in the interpretation of the formation. The use of 
the quadrat does not mean that the general methods of descriptive ecology 
are all to be discarded, whether they have value or not. The statement that 
quadrat methods are indispensable signifies merely that they must be used 
for research work in the development and structure of vegetation. They are 

1 Pound and Clements. A Method of Determining the Abundance of Secondary 
Species. Minn. Bot. Studies, 2:19. 1898. 



1 62 THE FORMATION 

not necessary in reconnaissance, nor do they displace general methods of 
real value. The use of the latter in even a supplementary way will grad- 
ually be discontinued, however, as fields become smaller by reason of in- 
crease in the number of workers, and as the need for precise methods 
becomes more universally felt. 

The quadrat constitutes the initial concept from which all the methods 
have grown. In itself, it has given rise to a variety of quadrats applicable 
to the most fundamental problems of vegetation. From it have come, on 
the one hand, the migration circle, and on the other, the transect. The 
latter .in turn has yielded the ecotone chart, and the layer chart. All of 
these are based upon direct and detailed contact with vegetation itself, and 
permit accurate recording of all the results obtained. 

QUADRATS 

199. Uses. In its simplest form, the quadrat, as the name implies, is 
merely a square area of varying size marked off in a formation for the pur- 
pose of obtaining accurate information as to the number and grouping of the 
plants present. As indicated above, it was first used for determining the 
abundance of the various species of a formation. This made it possible to 
ascertain the relative rank of the species of layers and formations, and 
enabled one for the first time to gain some idea of the minute structure of 
a bit of vegetation. The results were at once applied to the task of establish- 
ing a numerical basis for abundance, and of working out a new system of 
abundance to correspond. The quadrat method was also used, to determine 
the character of seasonal aspects, and to yield a knowledge of the exact 
differences in diverse areas of the same formation. Incidentally, the deter- 
minations of abundance were made the basis of an actual census of certain 
alpine formations. This, while it was extremely interesting to find that a 
square mile of alpine meadow contained approximately 1,500,000,000 plants, 
was confessedly destitute of ecological value. The most important applica- 
tions of the quadrat idea were made by Clements 1 in the chart, the perma- 
nent and the denuded quadrats. The development of these was due to the 
fact that zones or formations permit of comparison upon floristic as well 
as physical grounds, and that a detailed record of their structure is necessary 
for this purpose. Similar comparisons are necessary for the consocies, 
zones, and patches of the same formation, and the quadrat becomes an in- 
dispensable means for studying alternation and zonation. For the investi- 

1 The Development and Structure of Vegetation, 84. 1904. 
Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Neb., 
5:29. 1901. 



QUADRATS I 63 

gation of invasion year by year, and especially for succession, the method 
of permanent quadrats is imperative, and the denuded quadrat an invaluable 
aid. Changes, which would otherwise be incompletely observed and im- 
perfectly recorded, are followed in the minutest detail and recorded with 
perfect accuracy. 

200. Possible objections. The use of the quadrat has led to the criticism 
that it is needlessly detailed and thorough, and that, after all, the space 
covered is but a minute part of the entire formation. The first objection is 
one that has also been urged against the use of instruments of precision in 
the habitat. It is always brought forward by those who have not used in- 
struments, and as witnesses they are of necessity incompetent. No one who 
is familiar with the instrument or the quadrat by actual practice has felt 
that the methods based upon them were too thorough. In no case has the 
writer ever listed or mapped a quadrat without discovering some new fact 
or relation, or clearing up an old question. It can not be denied that quadrat 
methods require both time and patience, but this is true of any kind of re- 
search work that is at all worth while. Every ecologist, moreover, that 
has the interests of his field at heart and deprecates the present slipshod 
work, will appreciate the necessity of methods which seem like drudgery 
to the mere dabbler. 

The second objection, that the quadrat is at best but a small bit of the 
area under investigation, seems at first to be a valid one. It can not be 
gainsaid that the actual space studied is insignificant as compared with the 
whole formation ; still, it must be obvious that even a single quadrat can 
add at least some facts of value, which can never be obtained by the 
best of general methods. Furthermore, if the formation be an actual 
and not an imaginary one, a single quadrat will be in some measure repre- 
sentative. In the more homogeneous ones, it will have much the same 
value that a type specimen bears to the species established upon it. In 
formations which are less uniform, its value is correspondingly reduced, so 
that in formations which show marked zones, consocies, or patches, it 
becomes necessary to locate a quadrat in each. In the matter of representa- 
tion alone, the graphic method of the quadrat map with its close-focus detail 
photograph, is far superior to anything that can be obtained by the ordinary 
description and photograph. Finally, the scientific study and recording 
of succession, and particularly of competition, is an impossibility without 
the aid of the permanent and denuded quadrat. The stoutest champion of 
the practice of walking through a formation, and jotting down impressions, 
can not avoid their use if he would attack these problems, and, once familiar 
with the quadrat, his objections to the drudgery of thoroughness will 
soon vanish. 



164 THE FORMATION 

Kinds of Quadrats and Their Use 

201. Size and kinds. The unit size of quadrat is the meter, and when 
the term is used without qualification, it refers to the meter quadrat. To 
make them strictly comparable, and exactly divisible, unit quadrats are 
always grouped in squares; thus a major quadrat is a square of four units, 
and a perquadrat one of sixteen units, or four meters square. Quadrats of 
greater size are necessary in woodland and forest, where the rule, however, 
is that the woody plants alone are recorded fgr the whole quadrat, the her- 
baceous growth being listed or mapped for but one or two representative 
units. For special purposes, quadrats of 3, 5, 6, etc., meters may be used, 
but they are much less convenient. Quadrats are further distinguished with 
respect to their use. A list quadrat is one in which the plants are merely 
listed and the number of individuals of each species indicated. Chart 
quadrats are those in w r hich the area concerned is accurately mapped on 
plotting paper. Both list and chart quadrats are rendered permanent by 
careful labeling, so that their changes can be followed from year to year. 
The greater value of the chart causes practically all permanent quadrats 
to be of this type, and for the same reason only permanent chart quadrats 
are converted into denuded ones. 

202. Tapes and stakes. The lines for marking out quadrats are made 
of strong white tape, 5/8 inches wide. This is doubled and sewed firmly 
at both edges. Under moderate stretching, the tape is carefully marked 
off into decimeters, and eyelets 5 mm. in diameter are set in at each end 
and at the marks. This can readily be done by any shoemaker at slight ex- 
pense. The usual lengths are one and two meters, as these are most 
frequently used, and they can also be easily combined to make larger 
quadrats. The tapes are slightly longer than one meter in order that the 
distance between the end eyelets may be exact. The tapes of the larger 
forest quadrats should be divided into lengths of one meter, as these permit 
ready plotting and also make it possible to interpolate a meter quadrat for 
the study of the undergrowth at any point. The intervals of the tape are 
numbered fromjeft to right, as conspicuously and clearly as possible. For 
this a waterproof ink or paint is very desirable. For holding the tapes in 
position, hatpins, nails, and meat-skewers have been used with more or less 
satisfaction. The ideal stake, however, is one which holds the tape close 
to the ground, and can be readily moved. It is merely a stout wire, 3 mm. 
in diameter and 8 inches long, looped at the top, sharpened at the tip, and 
with a small ring of solder 3 inches from the tip. 



QUADRATS 



165 



203. Locating quadrats. In staking a quadrat, the end tapes are in- 
variably placed so. that the numbers read from left to right, and the side 
tapes so that they read down. In mapping, a fifth tape is stretched parallel 
to the top, and as each decimeter strip is marked, the outer tape is shifted 
to delimit the new strip. Indeed, the side tapes can be placed alone, and the 
plotting tapes moved down one at a time as the mapping proceeds, but it 
is usually more satisfactory to locate the quadrat exactly and to square it 
first, a task most easily done by enclosing the whole quadrat, and then using 
a fifth tape. In the case of list quadrats in open vegetation, the measuring 
strip is unnecessary, but as a rule it facilitates counting, as well as mapping. 








„< . 



Fig. 50. Mapping a major quadrat on Mount Garfield at 3,600 m. 



The List Quadrat 



204. Description. This, as the simplest form of quadrat, is employed 
primarily to ascertain the abundance of species in a formation or during 
a particular aspect of it. Since this can be obtained readily from the 
chart, the list quadrat has fallen more and more into disuse, except where it 
is desired to determine abundance alone, or to aid in deciding whether a 
chart is really representative. The size depends almost wholly upon the 
nature of the vegetation. When the number of trees is to be determined, a 
quadrat of 10 or 50 meters is necessary. In ordinary herbaceous forma- 
tions, the usual size is 2 meters, while the meter quadrat is used when the 



l66 THE FORMATION 

plants are especially small and crowded, as in alpine meadows. The loca- 
tion of the quadrat is based upon the general rule, but since its especial 
task is the determination of the greatest variable in vegetation, viz., number, 
it is necessary to use more quadrats, and to place them in areas which show 
the greatest differences in the mixture of species. For example, it was 
found that a half dozen list quadrats, when carefully located in the prairie 
formation, gave results almost identical with those obtainable from a larger 
number. With a little experience, the various degrees of mixture can be 
picked out superficially, and the corresponding number of quadrats es- 
tablished. If a single list quadrat is to be made for a formation or station, 
such a time should be selected as will make it possible to cover the greatest 
number of plants. Fortunately, this usually falls near the middle of the 
summer, when the remains of spring plants are still in evidence, and the 
autumn ones are sufficiently developed to be recognizable. In taking the 
census of different aspects, the quadrat should be made as near the middle 
of the period as is possible. 

205. Manner of use. In listing a quadrat, i. e., counting the individuals 
of each species, the plan followed is to list the smaller, less conspicuous 
plants first, since they are apt to be tramped down. As a rule, the outside 
tapes and the taller species afford sufficient landmarks. When this is not 
the case, the measure tape is used, and the individuals of all species are 
checked as they are found, while in the first method one species, rarely two, 
is taken at a time. • In cases of peculiar difficulty, it may be permissible to 
pull or break plants as they are counted, but ordinarily this can and should 
be avoided. Clusters, and bunches of stems from the same root are counted 
as single plants, and the number of stems indicated by an exponent. In 
the case of bunch grasses, each bunch counts as one plant. 

206. Table of abundance. The species are arranged in the final list 
in the order of their numerical importance, and are divided into groups 
which correspond to the different degrees of abundance. The latter 
are arranged in two series, based upon the fact that association is by 
groups* or by individuals. The table of abundance, based upon a 2-meter 
quadrat rather than upon the 5-meter one, by means of which the earlier 
results were obtained, is as follows : 

Social exclusive, no other species of vascular plants present 

social inclusive, above 100 

gr 1 gregarious 1 

gr 2 gregarious 2 

gr 3 gregarious 3 

sg subgregarious 

vg vfxgregarious 



100-50 


copious 1 


cop 1 


50-25 


copious 2 


cop 2 


25-10 


copious 3 


cop 3 


10- 5 


subcopious 


sc 


5- 1 


sparse 


sp 



QUADRATS I 67 

It is obvious that the above outline is faulty inasmuch as it takes no 
account of the height and width of the individuals. This is a serious 
defect, and it constitutes one of the many reasons why the list quadrat should 
be replaced by the chart quadrat. The prairie formation affords an un- 
usually striking illustration of this. A single quadrat may be filled by ten 
plants of Psoralea Horibunda, and at the same time contain 22,000 plants of 
Festiica octoilora. Yet the former is conspicuous and controlling; the latter 
plays an altogether insignificant role. This difference is readily shown by 
comparing a plant of each. The one is 3 x 3 feet, the other 3 x ^ inch. 
Such figures furnish a valuable check upon mere number, but make the 
brief, graphic designation of abundance difficult. An attempt has been 
made to solve this problem by roughly determining the space occupied by 
the plant, by means of the formula, height ( ir R 2 ) X abundance. This 
would give Psoralea a value of 210, and Festiica one of 1.6, which much 
more nearly represents their real importance in the formation. Abundance 
or numerical value is a floristic concept entirely, and has little place in 
ecology unless checked in the way indicated. The whole problem, ecologi- 
cally, depends upon an intimate knowledge of competition, and its solution 
in consequence is at present impossible. 

The Chart Quadrat 

207. Description and use. The detailed labor required in mapping makes 
it advisable to use the meter quadrat. An additional reason of much impor- 
tance is furnished by the desirability of securing a detail photograph of the 
quadrat. This is impossible with field cameras, which should not exceed 
63/2 x 8J^ inches, and are indeed most serviceable in the 4x5 size, if the 
area be larger. In open formations, the major quadrat of 2 meters can be 
used if necessary, but this is very rarely the case. Forest quadrats of ten 
meters square are easily charted, but detail photographs can not be made of 
them. Larger quadrats are impracticable ; they can be counted but not 
mapped to advantage. The location of the chart quadrat must be decided 
by the structure to be studied. Its greatest service is in connection with 
zones and societies of the same formation, which can be easily compared 
in the chart form. In fact, the chart quadrat may well be regarded 
as the fundamental method for inquiry into zonation and alternation. It is 
an important aid in delimiting areas from the contiguous formations, and in 
determining the relationships of mixed formations. It is also used to record 
the character of the different aspects, but this is done more satisfactorily by 
the permanent quadrat. 



1 68 



THE FORMATION 



208. The chart used is a decimeter square, and the scale is consequently 
io :i. It is outlined on centimeter plotting paper, and the centimeter squares 
are numbered at the edges to correspond to the intervals of the quadrat, i. e., 
the top and bottom lines are numbered from left to right, and the side lines 
from top to bottom. These outlines are ruled in quantity and used as needed, 
or the forms can be furnished by the printer. In practice, a special quadrat 
book the size of the chart has been used. The need of a second book may be 
avoided by outlining two charts on the plotting sheet, and filing the latter in 
the field record book. In the few cases where 2-meter quadrats are desir- 
able, four charts are used, care being taken to label them so that they can be 
combined whenever necessary. Ten-meter quadrats are recorded on the deci- 
meter chart also, each meter interval corresponding to a centimeter, i. e., the 
scale is ioo:i. 






Fig. f»l. Permanent chart quadrat, Andosacile, C are x -Camp anula- 
coryphium. 

209. Mapping is invariably begun at the upper left-hand corner of the 
chart, and is carried across the strip marked off by the plotting tape, deci- 
meter by decimeter. As soon as this strip is completed, a second one is 
formed by moving the top tape to a position one decimeter below the plotting 
tape, which then becomes the upper one. This is repeated until the last strip 
is reached. Little difficulty is experienced in locating each plant exactly, as 



QUADRATS 



169 



the decimeter interval is small, and the centimeter square which corresponds 
is divided into twenty-five tiny squares. Each plant is put in whenever pos- 
sible, but mats, turfs, and mosses are merely outlined in mass if the individ- 
uals are not distinguishable. This holds true of all large rosettes and mats, 
even when they are single plants. Symbols were formerly used for indicat- 



CC %. c^ a 




Fig. o2. Chart of the quadrat shown in figure 51. Legend: a, And- 
rosace chamaejasme ; c,Carex rupestris; t, Tetraneuris lanata; p, Poten- 
tilla rubricaniis; as, Arenaria sajanensis ; ar, Artemisia scopulorum; 
ag, Agropyrum scribneri; sa, Silene acaulis; st, Sieversia turbinata; d, 
Dasyphora fruticosa; al, Allium reticulatum; o, Oreoxis alpina. 

ing the various species. They have the advantage of requiring little space 
on the chart, and the disadvantage of necessitating constant reference to the 
legend. They are at present replaced by initials. By this plan, the decap- 
italized first letter of the generic name is used if no other genus found in the 
quadrat begins with the same letter. If, however, two or more genera begin 
with a, for example Agropyrum, Anemone, and Allium, the most abundant 



170 THE FORMATION 

one is indicated by a, and the others by the first two letters, as an, al. In 
case two species of the same genus are present, the species initial is used in 
connection with that for the genus, as ac and ar for Agropyrum caninum and 
Agropyrum richardsonii respectively. It is rarely necessary to exceed two 
letters for any species. Plants which regularly have several stems from the 
same root are indicated by the initial and an exponent as a 3 . Seedlings are 
represented by a line drawn through the letter. Usually the chart sheet 
affords sufficient space below the chart for the legend. When the list of 
species is long, the back of the sheet is used. 

210. Factors and photographs. Each chart is numbered, and the forma- 
tion, station, and date indicated. The constant factors, altitude, slope, and 
exposure are ascertained and recorded on the sheet. The variable factors 
are read in each quadrat whenever possible, and in addition to being pre- 
served in the record book, are noted on the chart sheet along with the base 
reading in the formation for the same time. This facilitates the interpreta- 
tion of the differences found when two or more charts are compared. Chart 
quadrats are regularly photographed. For this purpose a long- focus 4x5 
camera with a telephoto lens is used. At the proper distance this will make 
a view of the same size as the chart, thus making possible an exact compari- 
son of the two. The chart and photograph serve as mutual checks, as well 
as complements, since the former shows number, position, and arrangement, 
and the latter, height, form, position, and arrangement. The view is usually 
made by placing the camera directly in front of the middle of the lower tape, 
at such a distance that the side tapes fall just within the limits of the ground 
glass. The swing is always used in order that the focus may be uniformly 
sharp. Surface views of the quadrat can be taken by means of a device 
which permits the camera to hang downward from the tripod, or by means 
of a tripod with a swinging platform. Such views are especially valuable 
for the study of competition, since they give a clear idea of the spread and 
density of the various plants. They are difficult to make unless the vege- 
tation is low and nearly uniform in height. The usual photograph is much 
more serviceable in regular quadrat work. 

The Permanent Quadrat 

211. Description and uses. As stated heretofore, either list or chart quad- 
rats may be rendered permanent in order that they may be followed from 
season to season or from )^ear to year. As a matter of fact, however, an area 
which is to be studied repeatedly really demands charting, and in practice 
chart quadrats alone are made permanent. This is done simply by driving 



QUADRATS 



171 



a labeled stake at one corner of the quadrat, and locating the latter definitely 
in relation to a conspicuous landmark. When one is in residence for several 
years, practically all chart quadrats are converted into permanent ones, since 
the work already done in the chart quadrat is so much accomplished towards 
the permanent one. This is not necessary when one wishes merely to com- 
pare different areas of stable formations. As a rule, however, some change 
is constantly being wrought by invasion or competition, and the amount and 
direction of this can only be revealed by the permanent quadrat. The latter 
has a fundamental value for all kinds of invasion, but it is absolutely indis- 




Fig. 53. Permanent quadrat, Polygonile (Polygonum bistortoides) 
Ruxton Park; mapped and photographed July 22, denuded September 
8, 1903. 



pensable in studying complete invasion or succession, and in discovering and 
recording the gradual effects of competition. It is in the detailed investiga- 
tion of these dynamic phenomena that the paramount importance of the 
quadrat is most evident. If the experience of several years be taken as con- 
clusive, no other method is capable of revealing the minute changes as they 
are occurring. 

The permanent quadrat is regularly 1 meter square, a size determined both 
by the exigencies of charting and photographing. When ecograph batteries 
are used, the quadrat is located as close to the latter as is possible. Other- 



172 TPIE FORMATION 

wise, the quadrat itself should constitute a station for making factor obser- 
vations. This connection is absolutely essential, since the quadrat is used 
expressly to determine the structural changes, which are produced by physi- 
cal factors, and the reaction of vegetation upon them. Permanent quadrats 
are established in different formations or stages of a succession to trace the 
invasion of new species and the dropping out of old ones in response to com- 
petition. They serve to distinguish the proper formation, which represents 
a particular stage of development, from the mixed formations which precede 
and follow, and also to determine the exact course as well as the rapidity of 
the change that follows each reaction. When applied to different examples 
of the same stage, and to all the different stages of a succession, the whole 
development of the latter may be minutely traced and definitely recorded. 
The importance of following the changes from aspect to aspect is much less, 
since these are periodical rather than dynamic. They are an essential feature 
of structure, however, and it has been the practice to make at least one series 
of aspect charts from each permanent quadrat. 

For tracing the invasion and competition of lichens and mosses, which play 
a primary role in initial formations, a subquadrat is used. The size varies, 
but it is usually smaller than the quadrat, although the latter is entirely avail- 
able in the case of the large foliose lichens. For the crustose and smaller 
foliose forms, a subquadrat 2 decimeters square is used, and for the larger 
forms and tufted mosses, one of 5 decimeters. In the case of ground forms, 
tapes are employed, and the quadrat is permanently staked. On rocks and 
cliffs, where moss and lichen stages are most common, tapes are impracticable, 
and the quadrat is permanently outlined with paint. Charts of lichen quad- 
rats are made to the usual scale of 10:1. 

212. Manner of use. Permanent quadrats are mapped and photographed 
in exactly the same way as chart quadrats. As soon as this has been done, a 
labeled stake is driven at the upper left-hand corner, so that its edge indicates 
the exact position of the quadrat stake, and a smaller one is placed at the op- 
posite corner to facilitate the task of setting the tapes accurately in later 
readings. The label stake bears merely the number of the quadrat and the 
date when it was first established. It is firmly fixed and allowed to project 
just enough to enable it to be located readily. Its position requires careful 
landmarking when the quadrat is to be visited year by year. In forest for- 
mations, this is readily done by blazing, but in grassland it is necessary to 
have recourse to compass and pacing, or to erect an artificial landmark. 
After several charts have been made, a permanent quadrat attains a high 
value, and every precaution must be taken to prevent losing its exact loca- 
tion. At the second reading of a quadrat, whether in the succeeding aspect 



QUADRATS 1 73 

or year, the tapes are placed with reference to the stakes, and a chart and 
photograph are made in the usual manner. These are labeled and dated like 
the original ones, but they are numbered to indicate both the quadrat and the 
series, e. g., 15 2 indicates the second chart, and photograph made of quadrat 
15. The date indicates whether the readings are by the aspect or the year, 
though this may be shown also in the name of the series itself. It is clearly 
an advantage to have the two successive charts of a quadrat upon the same 
sheet, and to file all the charts and photographs of the same permanent quad- 
rat together, and in the proper order. 

Since much of the value of a permanent quadrat depends upon its use as a 
station for observing physical factors, it is unprofitable to establish a large 
number. The results of invasion and competition can be ascertained by the 
quadrat alone, but these should be merely preliminary to seeking for their 
causes. Clearly, a quadrat should be established for each battery of instru- 
ments, while additional ones should be located only in so far as they can be 
visited often enough to give an insight into the factors that control them. 
In view of the fact that the most important factors, water-content and light, 
are less variable than humidity, temperature, and wind, it will suffice if visits 
are made once a week. This is especially true when it is possible to refer 
the more variable factors to the continuous records of a base station. While 
all the results determined for permanent quadrats are preserved in the field 
record, a record of them is also kept on the reverse of the chart sheet for 
convenience in interpreting the different charts. 

The Denuded Quadrat 

213. Description. This is primarily a permanent quadrat from which the 
plant covering has been removed, after it has been charted and photographed. 
What is practically the same thing is obtained by establishing a permanent 
quadrat in a new soil, or in one recently laid bare and not yet reclothed with 
plants. These, however, are merely permanent quadrats, in which the first 
chart and photograph furnish a record of the habitat alone. They are of 
great importance in succession, and will be more fully discussed under ex- 
perimental vegetation. The denuded quadrat is of the usual size, I meter, 
though the smaller lichen quadrats are also denuded. The location is sub- 
ject to the conditions alreadv indicated, especially with reference to physical 
factors. The denuded quadrat, however, is particularly adapted to the study 
of invasion and the resulting competition. Consequently, when migration is 
markedly from one direction, a series of denuded quadrats throws a flood of 
light upon the actual steps in invasion. Denuding is a valuable aid in suc- 
cession, but it must be clearly recognized that, while permanent quadrats 



174 



THE FORMATION 



register the exact course of the succession, denuded ones can merely furnish 
facts as to the probable courses of stages not now in evidence. 



214. Methods of denuding and recording. Permanent quadrats may be 
denuded at any time during the time they are under observation. The best 
results, however, are to be obtained by establishing the two side by side, or at 
least close' together. In this way, they are mutually supplementary, and fur- 
nish the most evidence possible with regard to the procedure of invasion and" 
competition. Another advantage is found in that the same observations of 
climatic factors will do for both, though water-content and soil temperatures 



• '3SESS? 



. A* 



'Mfji 




Fig. 54. Denuded quadrat; this is the quadrat shown in figure 53; 
photographed September 7, 1904. 

are necessarily different. A quadrat which is to be denuded is first mapped, 
photographed, and labeled exactly like a permanent quadrat. The vegeta- 
tion is then destroyed. This is usually done by removal, though it may also 
be burnt, destroyed by flooding, or in some other manner. The method will 
depend upon the use which the quadrat is to serve. If it is to throw light 
upon the vegetation of an area in which denudation has affected the surface 
alone, the aerial parts only are removed by paring the surface with a spade. 
When the disturbance is to be more profound, the upper seed-bearing layer 
is removed, and the underground parts dug up. In the interpretation of a 



QUADRATS 175 

secondary succession, the denuding cause is made use of in a fashion as 
nearly natural as possible. Ordinarily, the plants are removed just below 
the top of the ground by a spade, leaving the underground parts undisturbed. 
This method has yielded very interesting results. 

Quadrats have been denuded in the fall after the majority of the plants 
have completed their growth. This is largely owing to the fact that other 
field work is less pressing at this time. Denudation can be done as well in 
the spring, though the invasion will be slower in this case, since the seeds 
which have accumulated will be partly or entirely removed. During the first 
season the denuded quadrat should be mapped every month, and, if the in- 
vasion be rapid, photographed also. In open formations, especially those of 
a xerophytic nature, a single chart and photograph made at the end of the 
season are sufficient. In a few cases of this sort, indeed, no invaders have 
appeared until the second year. Beginning with the second season, a single 
record taken near the close of the growing period will suffice. Denuded 
quadrats are labeled, dated, and filed exactly as other permanent quadrats,, 
but it should be noted that the first member of the chart and photograph 
series is that which records the original vegetation of the area denuded. 

215. Physical factors. When denuded quadrats are single, their physical 
factors must be observed in the usual way. If they are associated with per- 
manent ones, the ordinary readings are made for the latter, and those factors 
which are affected by exposing the soil are alone taken for the denuded area. 
These are the water-content, soil and surface temperatures, and in some sta- 
tions at least the humidity near the surface. As everywhere, water-content 
is the most important, but the temperature at or near the surface has a 
marked effect upon germination. Because of its bearing upon the latter, the 
surface water-content is usually determined also. This has been done by 
taking a surface sample 2 inches square and 1 inch deep. Denuded quadrats 
naturally show considerable differences from year to year as the action of the 
invaders becomes more pronounced. To this fact is due much of their value 
as aids in interpreting succession. 

Aquatic Quadrats 

216. 5cope. The preceding discussion of quadrat methods is based wholly 
upon their use in terrestrial formations. Wet meadow and dry bog are the 
wettest places in which quadrats have been used. It is clear, however, that 
with certain necessary modifications, quadrats can be used as successfully-, 
though not as conveniently, in many water formations as in land ones. The 
tapes need to be raised above the surface of the water by longer stakes, and 



Ij6 THE FORMATION 

photographs often taken from a boat, but otherwise the usual methods apply, 
at any rate for bogs and shallow bodies of water. In lakes or streams the 
tapes might be attached to buoys or floats. The determination of factors is 
made as usual. Permanent quadrats are feasible in many cases at least, and 
denuded quadrats are not altogether impossible. 

transects 

217. The transect is essentially a cross section through the vegetation of a 
station, a formation, or a series of formations. It is designed primarily to 
show the order of arrangement of species in zones and societies, but it also 
serves as a record of the heterogeneity of any area. In the form of the layer 
transect, it furnishes a graphic method of representing the spatial relations 
of the species in layered formations, e. g., forests, ponds, and lakes. It is 
merely a logical extension of the idea underlying the quadrat, and the tran- 
sect is, indeed, little more than an elongated quadrat. An important differ- 
ence, however, lies in the fact that the former normally traverses areas more 
or less unlike, while the latter is always located in a homogeneous one. Fur- 
thermore, the transect is plotted with especial reference to the topography. 
With respect to dimension, transects are classified as line, layer, and belt 
transects, and the latter may also be permanent or denuded. 

The Line Transect 

218. Description and method. A simple transect is sometimes made by 
establishing the points between which it is to be run, and then recording the 
plants pace by pace along this line. This is satisfactory where the striking 
changes in structure are desired. A more accurate method is ordinarily used, 
since it gives detailed results, and at the same time brings out the more gen- 
eral features. For this, use is made of a tape of proper length which is 
divided into decimeters. Tapes of 10, 50, and 100 meters are used, and if 
they are furnished with eyelets, transects of intermediate lengths may be run 
with them. When longer transects are desired, as in the case of forest for- 
mations, tapes of 500 or 1,000 meters should be used with eyelets a meter 
apart. The transect is located in the area to be studied by running the tape . 
from one landmark to another, fastening it here and there by means of quad- 
rat stakes. Previous to this, the shortest distance between landmarks is as- 
certained when the transect runs through a depression or upon a level surface. 
In the case of an elevation, the height is ascertained by a barometer, the 
length and angle of the two slopes obtained, and the length of the base line 
determined from these data. The field record of the arrangement of the 



TRANSECTS 



177 



**€*, 



^' 



\<3 



#L 



SS^.^ — 



Fig. 55. Line transect running east 
and west in the Picea-Pinus-hylium, 
showing the relation of the herbaceous 
layer to the Carex-Catha-helitim, in- 
vading along the brook; ecotones at e. 



plants is made entirely without reference 
to the surface line. The vertical lines on 
the centimeter sheet are taken to corre- 
spond with the tape, and the individual 
which touches the latter on either side is 
recorded to the right or left respectively 
and within the proper square. The spe- 
cies are indicated as for quadrats. A 
single row on either side may be taken 
alone, but the double series serves as a 
desirable check. After the record is 
made, the topography of the transect is 
drawn carefully to scale. This drawing 
is made upon the scale of 100:1 for 
transects of 10 meters or less, and of 
1000:1 for those that are longer. The 
combination of this drawing with the line 
series of plants can not be made advan- 
tageously in the field. For the shorter 
transects, meter sizes of centimeter plot- 
ting paper can often be used to advan- 
tage. In this event, the topographic line 
is drawn to the scale of 10:1 and the 
series of plants transferred directly to it. 
In the case of transects between 10 and 
100 meters, the scale of the drawing is 
increased from 1000:1 to 100:1, so that 
each decimeter of the original series is 
compressed into a centimeter. For the 
longest transects, corresponding reduc- 
tions must be made, but in these it will 
be remembered that the series is plotted 
by meter instead of decimeter. 

219. The location and size of line 
transects are determined by the purpose 
for which they are designed. Short 
transects are valuable for detail, but they 
can be used to advantage only where 



178 THE FORMATION 

changes in arrangement are taking place rapidly. They are especially 
adapted to the study of minute alternations and to the zonation of small 
ponds, streams, ditches, roads, blowouts, etc. Longer transects can not fur- 
nish the same detail, on account of the amount of time necessary, but they 
are invaluable for the zonation and alternation of larger areas, such as the 
consocies, formation, and formation series. They are of particular impor- 
tance for the record of zonation, since they afford a clue to the topographic 
symmetry of the area. The location of a transect depends upon the area to 
be studied, though it should always run through a portion as typical as pos- 
sible. The general direction is ascertained by. means of the compass, and 
when there is a measurable difference in elevation it is taken by the barometer 
or otherwise. 

The points at which ecotones cross the transect are carefully indicated 
upon the chart. They serve as stations for simultaneous readings of phys- 
ical factors, though in the majority of cases water-content readings alone will 
determine the reason for the ecotone. Photographs of line transects should 
be made while the tape is in position, in order that the superficies of the series 
may be as evident as possible. 

The Belt Transect 

220. Details. This differs from the line transect in that it is wider, and 
consequently affords a more accurate record of the. arrangement of plants. 
While both give the actual facts of distribution, the line transect necessarily 
ignores the minor lateral deviations in position. These are brought out in a 
strip of some width, and the belt transect thus gives a more correct view of 
the variations which result from competition in an area physically homo- 
geneous. The width of such transects depends upon the length, and the 
character of the vegetation. The standard width is one decimeter in herba- 
ceous formations, and one meter in the long transects which are used in 
woodlands. In open vegetation, especially in the initial stages of succes- 
sions, the width may often be increased to advantage, but ordinarily the 
amount of work necessary to run a belt transect of some length limits the 
width to one decimeter. 

The location of a belt transect, the choice of landmarks, the determination 
of direction and elevation are made exactly as for the line transect. The 
topographic map is made in precisely the same way also, the scale used de- 
pending upon the length. Two tapes, however, are employed, and these are 
placed so that they mark off a strip just one decimeter wide. Every few 
meters, or oftener if need be, they are checked by a decimeter rule, and fixed 
firmly in place by quadrat stakes. The arrangement of the plants is recorded 



TRANSECTS 179 

as for the line transect, except that the record covers a decimeter strip just 
as in quadrat work. Accordingly, an interval of a centimeter is left on the 
sheet between the successive portions of the strip, in order that the latter 
may be put together without confusion when the topographic map and the 
plant series are combined. The record should invariably start in the upper 
left-hand corner and read down. The map and the centimeter strip record- 
ing the plants of the transect are combined on a common scale as already 
indicated for the line transect. 

The ecotones of zones are shown on belt transects by single cross lines, 
and those of consocies by parallel cross lines. In taking photographs of the 
transect, it is desirable to use guidons to mark these points clearly. The 
same device may also be used to indicate the course of the transect, when the 
tapes are completely hidden by the plants. Physical factor readings should 
always be taken, and, as before, they are best made at the intersections of the 
ecotones. 

The Permanent Transect 

221. Advantages. Both line and belt transects, after they have been re- 
corded, should be rendered permanent, in order that they may serve to indi- 
cate the changes of a heterogeneous area from year to year in the same 
detailed fashion that the permanent quadrat does for homogeneous ones. 
For historical as well as for physical reasons, the ecotones of zones and of 
consocies are subject to change from year to year, and the amount and di- 
rection of this change can only be ascertained from annual records made in 
exactly the same spot. By means of the permanent transect alone the very 
origin of such areas can be followed from one stage to another of the suc- 
cession. Moreover, the transect is equally valuable with the quadrat .in 
making it possible to follow every step of the minute changes wrought by 
competition. 

222. Details. The transect is made permanent by blazing the landmarks 
at either end, if these already exist, or by erecting them when it is necessary. 
A label stake is driven at each end, on which is painted the number and date 
of the transect and its length. Each stake should also indicate the exact di- 
rection in which the other lies. The position of the ecotone is indicated by 
smaller stakes bearing the number of the transect and the date when the 
ecotone was found at that point. These are left in place, and in a few years 
show very graphically the change in position of the zones. For the first 
season, permanent transects afford results of great value when recorded for 
each aspect, but after this an annual visit will suffice. The details of map- 
ping, plotting, etc., are identical with those indicated above, with the addition 



l80 THE FORMATION 

that all charts and photographs must bear the number of the reading as well 
as that of the transect. Physical factor observations are taken as often as 
the charts are made, and the results noted on the back of the chart sheet for 
purposes of ready comparison. 

The Denuded Transect 

223. The denuded transect bears exactly the same relation to a perma- 
nent one as that which exists between the denuded and the permanent quad- 
rat. While the permanent transect records the actual mutations due to 
changing physical factors or to competition, the denuded transect throws 
needed light upon the mobility and ecesis of the various species, and upon 
the nature of the competition between them. Denuded transects may be 
established wherever it seems desirable, after the strip has been properly 
charted and photographed. The most valuable results, however, are secured 
by locating each one alongside of a permanent one. The best plan is to lo- 
cate and chart two permanent transects a meter apart. A single view is then 
made of the two. One of them is denuded together with a strip 2 decimeters 
on either side, resulting in a denuded transect 5 decimeters wide. In chart- 
ing this during succeeding years, the entire width may well be plotted as 
long as the vegetation is open, but after it has again become well established, 
it is necessary to save time by confining one's attention to the central deci- 
meter strip. Photographs can be made either of the permanent and denuded 
transects singly, or of the two together. The latter method has certain ob- 
vious advantages. Climatic factor readings can be made for both transects 
in common, but all those factors which are affected by the exposure of the 
soil surface must be observed in each. 

The Layer Transect 

224. This is a modification of the line transect, by means of which the 
vertical relations of plants are also shown, especially the tendency to form 
layers which is so regular a feature of forest formations. Owing to the dif- 
ficulty of charting in three planes, belt transects do not lend themselves to 
this purpose. Because of the greater complexity, layer transects can rarely 
exceed ten meters in length except in those formations where layering is lit- 
tle or not at all developed. The simplest method is to establish a line tran- 
sect in the ordinary way, and then to record the height of each plant as its 
position is noted. This is done by means of a measuring stick ruled in deci- 
meters, which can be moved from interval to interval along the tape, or bet- 
ter, by two such sticks connected by tapes a meter long at every five deci- 



ECOTONE CHARTS l8l 

meters of the sticks. These should be two meters high for woodland, and 
one meter for grassland. Layer transects often run on even surfaces, but if 
this is not the case, the usual data for a topographic map should be taken. 
The final chart is constructed on the scale of 10:1, the height of each plant 
being indicated by a vertical line equal to .1 of the observed height. A pho- 
tograph of a representative meter of the transect is taken when the measur- 
ing sticks and rods indicated above are in position. Physical factor read- 
ings, principally of light, but often also of humidity, temperature, and wind 
are made at the height of the various layers when these are present. 

ECOTOXE CHARTS 

225. The contour lines of zones and consocies are of the utmost impor- 
tance in recording the structure of vegetation. They do not permit such 
accuracy as do quadrats and transects, but this is hardly to be considered a 
disadvantage in view of the fact that ecotones are rarely sharply defined. 
In establishing the ecotones of zonation, the width and the length of the 
base, i. e., the area of excess or deficiency, or as much of it as is to be con- 
sidered, are determined. This base may be road, ditch, pool, lake, or stream, 
or the peak or crest of a hill, ridge, or mountain. When the zonation is 
bilateral, meter tapes are run at right angles to the base, at proper intervals, 
and the points and the distances where the ecotones cross are noted. In the 
case of radial zones, the tapes are run in the four cardinal directions, and 
if the base be large, in the four intermediate ones also, the intersections 
being likewise noted. From the data thus obtained, the zones may be out- 
lined with a fair degree of accuracy. If the series be an extensive one, it is 
charted to the scale of 100:1 ; in cases of small areas, however, the scale of 
10:1 will give better results. Whenever the zones show clearly enough to 
warrant, a photograph is also taken. Water-content readings are of para- 
mount importance in the interpretation of zones. Samples should be taken 
at all intersections, and the resulting values indicated at the corresponding 
points upon the chart. When the zones are broken up into alternating 
patches in consequence of asymmetry in the topography, the ecotones of the 
latter are traced in a similar fashion from the center of each as a base, 
the absolute position of which is ultimately determined with reference to 
the ecotone lines already established. 



1 82 THE FORMATION 

THE MIGRATION CIRCLE 

226. Purpose. The migration circle is designed to record the invasion 
of species, since it operates outward from an individual or a group of plants 
as a center. As migration takes place to a certain degree in all directions, 
a circle is better adapted to the purpose than the quadrat. From the very 
nature of invasion, migration circles should always be permanent in order 
that the yearly advance may be accurately noted. Circles of this character 
are important aids in the study of any vegetation, except, perhaps, one that 
has practically become stabilized. Their great value, however, is found 
in succession, where it is necessary to trace the movement of new individuals 
away from the original invaders as centers of colonization. 

227. Location and method. The size of the migration circle is largely 
controlled by the density of the vegetation, and in some degree by the height 
of the species also, since this determines the trajectory of the disseminule. 
In close formations, a circle of I-, rarely of 5-meter radius can best be used, 
but in the more open initial stages of succession a radius of 5, 10, or, in ex- 
ceptional cases such as open woodland, even 25 meters, affords the best 
results. The location should always be made with a plant or group of 
plants of the species to be studied as a center. This migration circle differs 
from the quadrat jn that it is used to show the movement of one, rarely 
two or three species, and not the position of all the plants within it. The 
center is permanently fixed by driving a labeled stake with the number of 
the circle and the data. Two tapes the length of the radius are used for 
recording. These are provided with the usual eyelets, 5 decimeters apart, 
and are fastened on a peg in the top of the central stake so that they move 
readily. At the outer ends they are staked 5 decimeters apart by a tape of 
this length when the radius is 1 meter, and 1 meter when the radius is 5 or 
10 meters. The record forms must be especially prepared on blank sheets 
about 9 inches square. The scale is 10:1 for circles of 1 meter, and 100:1 
for those of 5- and 10-meter radius. In the former, concentric circles are 
drawn about the center at intervals of 5 decimeters, and radii are drawn 
to the circumference at the same interval. In the larger circles, the inter- 
vals are 1 meter. Each segment of the circle is read by means of the two 
tapes, and the position indicated with reference to the concentric lines and 
radii. When but one species is read, a tiny circle is used to denote the posi- 
tion of each plant. If more than one is used, the symbols are those already 
indicated for the quadrat. One tape is left in place and the other with the 
segment tape is shifted to a new position, and the resulting segment is read 
as before. The exact position of the base radius is fixed by a label stake, 



CARTOGRAPHY 1 83 

in order that the segments of successive years may exactly correspond. 
The record sheet is labeled, dated, and filed. By folding at one edge, it may 
be filed in the regular field book. 

228. The denuded circle is established in the same way as. a permanent 
one. The original position of the individuals of the species under considera- 
tion may be recorded or not, depending upon the use to be made of the 
results. The safest plan is first to read the circle in the usual way, and 
then to denude it. The latter should be done in such a way as to remove 
all the disseminules from the surface in so far as possible. It is essential 
also that this be done before the seeds are mature and begin to be scattered. 
The central plant or cluster is of course not removed. In special cases, all 
the plants of the species are allowed to remain to serve as centers of coloniza- 
tion. The successive yearly readings of the denuded area are made exactly 
as for a permanent circle. Permanent and denuded circles, like quadrats, 
should always be established near each other so that they permit of ready 
comparison under similar conditions. 

229. Photographs of migration circles furnish the most detail when 
the camera is placed just behind the central group in such a way as to show 
its relation to the other individuals or clusters of the circle. In the denuded 
circle, or when the plants stand out conspicuously from the bulk of the 
vegetation, it is not necessary to use guidons, but in other cases the latter 
greatly increase the value of the picture. Factor readings are less im- 
portant for migration circles than for quadrats and transects. The factors 
of principal importance are those that deal with migration and ecesis, i. e., 
wind, water-content, and soil temperatures. The former may be determined 
for both circles in common, but the conditions that affect ecesis must be 
observed separately for each. 

cartoCtRAPhy 

230. Value of cartographic methods. Chart, map, and photograph are 
records indispensable to the systematic study of vegetation. They serve 
not merely to preserve the facts ascertained, and to permit their ready com- 
parison, but they also put a premium upon accurate methods, and conse- 
quently bring to light many points otherwise overlooked. For ecology, 
they have the value which drawings possess in taxonomy, in that they 
make clear at a glance what pages of description fail to indicate. They 
are the fundamental material of comparative phytogeography, and in all 
careful vegetational study their use is no longer optional but obligatory. 



I84 THE FORMATION 

Hence it is obvious that cartographic methods should be clear and simple, 
and that they should be uniform, so that charts and maps of widely separated 
formations may be directly compared without difficulty. It is not to be ex- 
pected that uniform methods will come into general use immediately, but 
a proper appreciation of the obligation that rests upon every ecologist to 
make his results both easily comprehensible and usable will serve to produce 
this very necessary result. In the treatment that follows, as elsewhere, no 
attempt is made to describe the general cartographic methods used by other 
ecologists, notably Flahault. The methods employed by the author form a 
complete system, which has proved valuable, and for various reasons it 
alone is discussed here. 

231. Standard scale. The question of the scale to which charts and 
maps are to be made is of primary importance. The general principle is 
that the ratio between area and drawing should be as small as possible. 
Moreover, charts and maps of the same character should always be drawn 
to the same scale, unless a good reason to the contrary exists. The ideal 
scale is 1:1, which is manifestly an impossibility. This is approached most 
nearly in the quadrat chart where the scale is 10:1. Charts of definite 
areas are made on a scale as large as possible, while maps of formations, 
regions, etc., are necessarily drawn upon a very small scale. General maps 
designed to show the distribution of species and formations, or the vegeta- 
tion of continents, are usually not drawn with reference to a scale at all. 
While it is manifestly impossible to use the same scale for charts and maps, 
it is feasible and desirable that they be constructed upon scales readily con- 
vertible into each other. This is most satisfactorily accomplished by means 
of the decimal system, and the various type scales are 10:1, 100:1, 1000:1, 
etc. The first two or three scales are used for charts of quadrats, transects, 
and circles ; the remaining ones are employed in making maps of large areas. 
No attempt has been made to draw an absolute line between charts and 
maps, but an endeavor is made to restrict the term chart to the record of 
the number and position of plants, while maps deal with the arrangement 
and location of formational areas. It is hardly necessary to point out the 
reasons why all charts and maps should be based upon the decimal system of 
scales. Experience will furnish the very best of arguments. 

232. Color scheme. The first requisite for the graphic representation 
of formations, regions, etc., is that each class of formations be invariably 
indicated by the same color. It is also necessary that the colors and shades 
be easily distinguishable, and it is at least desirable that they be referred to 
the different classes in some consistent sequence. Uniformity in all these 



CARTOGRAPHY 



185 



points is greatly to be desired at the hands of all ecologists. Here, as in 
the case of the standard scale, uniformity will be found the more desirable 
the more impossible it is made by ignoring it. In the use of color to repre- 
sent regions and provinces, on maps too small to indicate formations, the 
color of each division is represented by the color of its dominant forma- 
tion; thus the prairie province is colored ochroleucus on account of the 
color used to represent prairie formations, the boreal-subalpine zone 
atrovirens on account of the typical coniferous forests, etc. No endeavor 
has been made to take account of the various types of formations, e. g., the 
different coniferous forests, as this is a problem to be worked out for more 
local maps in various shades of dark green, etc. The following color scheme 
which has been based upon the points made above is proposed as a satis- 
factory solution of the problem. The color standard used is that of 
Saccardo"s Chromotaxia. 

I. Hydrophytic Formations : blue 

1. Marine: cyaneus 

2. Brackish : ardesiacus 

3. Freshwater : caeruleus 

4. Swamps and marshes : caesius 
II. Mesophytic Formations 

A. Forest formations : green 

1. Coniferous forests: atrovirens 

2. Broadleaved evergreen forests : viridis 

3. Deciduous forests : Havo-virens 

B. Grassland formations : yellow 

1. Meadows : melleus 

2. Prairies : ochroleucus 

C. Culture and waste formations: red 

1. Fields : ruber 

2. Groves and orchards : atropurpureus 

3. Wastes : purpureas 
III V Xerophytic Formations : brozvn 

1. Deserts : isabellin us 

2. Plains and steppes : avellaneus 

3. Saline formations : umbrinus 

4. Arctic-alpine formations : testaceus 

233. Formation and vegetation maps are detailed maps of a single 
formation or a series of them, showing the formational limits, and when 
the scale is not too small, the ecotones of zones and consocies. In the cases 



I 86 THE FORMATION 

where the topography is level, as sometimes happens in mapping single 
formations, the chain and pedometer must be used to ascertain the size of 
the different areas. Indeed in all mapping of vegetation, the methods of 
surveying are directly applicable. Over large areas, however, it is not neces- 
sary that limits be drawn with mathematical accuracy, and for the purposes 
of the ecologist, the plane table and camera are satisfactory substitutes for 
the surveyor's transit, at least in the present aspect of the subject. When 
the formation or group of formations is commanded by an elevation of some 
height, the latter is used as a base. A plane table is established upon it and 
the topographical and vegetational features are recorded in the usual way. 
This map is usually supplemented by a series of views from the same base. 
Indeed it has come to be recognized that a complete series of photographs 
of this kind give a more valuable record than the plane table, and that the 
construction of an accurate map from them is an easy matter. Since the 
camera saves much time and energy also, it is used almost exclusively to 
furnish the data for map making. In hilly, and especially in mountainous 
regions, the photographic method is indispensable. Its application is ex- 
tremely simple. A central hill or mountain is selected, and from it a series 
of views is taken so that the edge of one exactly meets the edge of the other. 
This is an extremely important matter, and demands much nicety of judg- 
ment. The camera is kept in the same spot, and after each exposure it is 
turned as the operator looks through it until a landmark at one edge just 
passes from view at the other, As soon as the new position is determined, 
the tripod screw is turned to hold the box firmly in position. In case of a 
slight jar, the exact position should again be obtained. If the series is 
accurately made, the resulting prints will give a complete panoramic view 
of the region, without overlap or omission. For this purpose, a 6 l / 2 x 8^2 
camera is desirable, since the topographic and vegetational features are 
larger and stand out more distinctly. A large camera requires fewer changes 
of position, and hence saves time and reduces the chance of error. A 
4x5 camera serves the purpose sufficiently well, though it requires a little 
more care in operation on account of the greater number of exposures neces- 
sary. This may be avoided in some degree by the use of a wide-angle lens 
if the depth of the area is not too great. Whatever camera may be used, a 
telephoto lens is a very desirable adjunct, since it enables one to choose be- 
tween three different sizes of the view without changing the position of the 
camera. To avoid possible confusion, the exposures are always made from 
right to left, and the plates are used in the numerical order of their holders. 
For the same reason the landmarks are described and numbered in their 
proper order. The prints obtained are mounted on a card in sequence. 
The view map may be preserved in this form, or it may be reduced or 



CARTOGRAPHY I 87 

enlarged by making a copy to the size desired. Outline maps of 
topography may be traced from the resulting negative, and the formations 
rilled in by means of the proper colors. The most satisfactory method, 
however, is to have the original views or the copy printed "light" and to 
color the formations just as they appear there, with all the wealth of topo- 
graphic and vegetational detail. If a detailed topographic map alone is 
desired, this is traced directly from the large copy. 

234. Continental maps. A method of determining the general out- 
lines of regions, provinces, and vegetational zones as a preliminary to their 
detailed study has been used successfully for several years. 1 This is based 
upon provincial and continental maps on which are traced the geographical 
areas of the species of genera typical of the various formations. Detail 
topographic maps of the prairie province and the North American continent 
have been used for this purpose. A number of the facies of extensive and 
representative formations of the different portions of the continent are 
selected and grouped according to genera. One map is devoted to each 
genus, unless the number of species is large. In this case a number of maps 
are used, since the limits are apt to become confused. The range of each 
species is determined from all the reliable sources, and a corresponding line 
is drawn upon the map to delimit its geographical area. The limits of the 
area of each species are drawn in a different color, and the name of the species 
printed in the same color in the legend. Although this work has as yet been 
done only for the trees of North America, and for the grasses and principal 
species of the prairie province, it promises to constitute a final method for the 
limitation of vegetational divisions. It is clear that if the original data 
concerning ranges are accurate, the increasing study of formations will do 
little more than rectify the detailed course of the limiting- line, since in most 
cases facies and formations coincide in distribution. The limiting line or 
ecotone of a zone or province is a composite obtained from the limits of 
certain representative facies and principal species, and checked by the limits 
of species typical of the contiguous vegetations. Thus, the boreal-subalpine 
zone, is clearly outlined by combining the limits of Popidus tremidoides, 
Larix americana, Pmus banksiana, Abies balsamea, Picea mariana, Picea 
canadensis, and Betula papyracea, and checking the results by the areal limits 
of the hardwoods and grasses to the southward. 

1 Pound and Clements. The Vegetation Regions of the Prairie Province. Bot. 
Gaz., 25 :381. 1898. 



1 88 THE FORMATION 

PHOTOGRAPHY 

235. The camera is an indispensable instrument for the ecologist. Al- 
though it has too often been employed to give an air of thoroughness to 
work of no ecological value., it is as important for recording the structure 
of vegetation as the automatic instrument is for the study of the habitat. 
No ecologist is equipped for systematic field investigation until he is pro- 
vided with a good camera and has become skilful in its use. For this reason, 
it is felt that a few hints concerning photographic methods and their appli- 
cation in ecology may not be out of place. No written advice can take the 
place of experience, but certain elementary suggestions and cautions will 
greatly shorten the apprenticeship of one who does not have the good for- 
tune to be taught by a professional photographer. To the student of 
ecology, the camera is not a toy. It must be understood and operated with 
as much thoroughness as any other instrument, and when this is done, the 
results will be equally certain and desirable. 







Fig. 56. 4x5 long focus "Korona" camera (series V). 

236. The camera and its accessories. Although two cameras are desir- 
able whenever it is possible to obtain them, a single one will meet all the 
requirements of field work. This should be 4x5 inches in size, since it is 
much more convenient and will do all the work that a larger camera can. In 
the comparatively few cases in which larger views are needed, the 4x5 
negatives can be readily enlarged. The smaller instrument is less expensive 
in operation because of the cheapness of the plates, and it gives a negative 
of the proper size for lantern slides and for reproduction. A 6}^. x 8^2 
camera is valuable in special cases, such as making a series of photographs 
for maps. In the writer's own experience, the 6V 2 x 83/2 camera, although 
iised exclusively at first, has been almost completely supplanted by the 
4x5. The best field camera is of the folding type with a good stout box. 
It must be what is known technically as a long-focus instrument, which 



PHOTOGRAPHY 



189 



enables small objects to be taken natural size and permits the use of a 
telephoto lens. It should be provided with a swing and also a reversible back 
by which the position of the plates can be changed instantly. The lens must 
be of the telephoto pattern, which makes it possible to use the front or 
back lens either alone or in combination. The chief advantage of this is 
that the image, when distant, may be made of three different sizes without 
changing the position of the camera. Generally speaking, the high-priced 
rapid lenses are the best, since it is exceptional to get the desired length of 
exposure in vegetation, on account of the ease with which the plants move 
in the wind. Before buying such a lens it is desirable to test its rapidity 
and depth of focus, since it is not necessarily better than some of the lenses 
furnished with good cameras. The lens should be provided with an iris 
diaphragm capable of being stopped down to 128 or 256. The shutters 
furnished with the ordinary lenses are satisfactory, since "snap-shots," 




Fig. 57. 5x7 long focus "Premo" camera. 



i. e., instantaneous exposures, are practically never possible for plants. The 
automatic shutter of the "Prerno" camera is an especially convenient form. 
All shutters should be carefully tested before using to determine the exact 
time value of the exposures indicated. It is not uncommon for the exposure 
at 1 second, or at other points, to have a value quite different from the one 
indicated. When this is the case, it is evident that it can not be known too 
soon. The camera should have at least a half-dozen double plate-holders. 
These are numbered consecutively so that the figure uppermost when the 
holder is in the camera will indicate the number of the plate exposed. A 
carrying case is desirable on a long trip when all the plate-holders must 
be taken, but ordinarily it is a disadvantage, since the camera box will carry 
two or three holders. The camera cloth should be as small and light as 
possible, and at the same time opaque. The most satisfactory one for the 



190 



THE FORMATION 



field is the rubber cloth. The tripod should be a happy combination of 
lightness and stability, a condition more nearly reached by the aluminum 
tripod than by any other. It should have not less than three joints in order 
to facilitate the use of the long- focus upon objects near the ground. 

237. Choice of a camera. There is not a great deal of choice between 
the moderate-priced cameras of the various makers. A field camera is re- 
stricted to certain special uses, and hence is more serviceable when attach- 
ments useful only in portraiture or instantaneous work are absent. Even the 
ray filter, which has some value in the indoor photography of flowers, is use- 
less in the field on account of the long exposure required. From consider- 
able experience, "Premo" and "Korona" cameras have been found to be 
very satisfactory instruments. Doubtless the same statement would be 
found true of all the standard makes, but they have not been used by the 




Fig. 58. 5x7 "Korona" Royal camera. 



writer. "Premo" cameras are made by the Rochester Optical Co., Rochester, 
N. Y., and "Korona" cameras by the Gundlach-Manhattan Optical Co., 
Rochester, N. Y. When two or more cameras are used, the best results can 
be obtained if they are of the same make, since the details of operation are 
then the same. The reduced liability of making a blunder is often offset 
by the fact that a different pattern will permit of a wider range of use. 
Any standard brand of plates will produce good negatives when skilfully 
used ; at least, this has been proved in the case of the Cramer, Hammer, 
Seed, and Stanley brands. Every professional photographer has his favorite 
brand of plate, but the ecologist will do well to give the various kinds a 
thorough trial, and then to invariably use the one which gives him the best 
results. Thus, while it seems to be less popular with the profession than 
the others mentioned, the writer has obtained at least as satisfactory results 



THOTOGRAPHY I9I 

with the Stanley plate as with the others, and consequently now uses it 
exclusively, since it is cheaper. The one important point is to make a final 
choice only after personal experience, and then to always use plates of the 
same brand, and preferably of the same rate of speed. 

238. The use of the camera. To the ecologist, objects to be photo- 
graphed fall into two categories, viz.,' those that move, and those that do 
not move. For practical purposes, areas sufficiently distant to render the 
movement imperceptible belong to the latter, as well as those, such as rock 
■ lichens, many fungi, etc., which can not be stirred by ordinary winds. The 
treatment accorded the two is essentially different. A fundamental rule of 
ecological photography is that detail must receive the first emphasis. The 
ecological view should be a picture as well as a map, however, but when one 
must be sacrificed, artistic effect must yield to clearness, and accuracy, i. e., 
technically speaking, contrast must give way to detail. ■ Leaving apart the 
necessity of securing a sharp focus, which holds for all work, detail or defi- 
nition depends directly upon the aperture of the diaphragm. Detail is in- 
creased by decreasing the size of the aperture. This in turn increases the 
length of time necessary for a proper exposure, and consequently the danger 
that the plant will be moved in the midst of the exposure. When the move- 
ment is negligible, the invariable rule should be to reduce the aperture to its 
smallest size, and to expose for a corresponding time. In all cases where 
the plants are close enough to show even a slight blurring on account of the 
action of the wind, the time of exposure must be reduced, in the hope that 
a short period of quiet will suffice for it. This reduction in time must be 
compensated by increasing the aperture of the diaphragm, and hence the 
amount of light which strikes the plate. The proper balance between the 
two is a matter of considerable nicety. It depends much upon the vagaries 
of the wind, and can readily be determined only after considerable ex- 
perience. Although regions naturally differ somewhat in the nature of their 
winds, much experience in prairie and mountain regions warrants the 
primary rule that views of vegetation and plants subject to movement are 
not to be attempted on windy or cloudy days when it can possibly be avoided. 
Even on reconnaissance, a poor picture is no better than none at all, while 
in resident' work a time will come sooner or later which will permit the 
making of a view satisfactory in all respects. There may be occasional in- 
stances when one is rewarded for keeping the camera trained on a particular 
spot for hours, and for wasting several plates in the hope that still mo- 
ments will prove to be of the requisite duration. As a regular procedure, 
however, this has nothing to commend it. 

Various methods have been tried to reduce or eliminate the trouble caused 
by the wind. Canvas screens have been used for this purpose with some 



192 THE FORMATION 

benefit. When the picture is worth the trouble, a tent may be erected to 
afford a very efficient protection. This is too prodigal of time and energy, 
however, to be practicable under the usual conditions. Flashlight exposures 
on still nights are sometimes feasible, but the disadvantages connected with 
them are too great to bring them into general use. The best procedure is 
to bide one's time, and to take quadrats, transects, and other detail areas, 
as well as many plant groups, at a time that promises to be most favorable. 
Single plants can often be moved in the field so that they are protected from 
the wind, or so that they are more strongly lighted. Slender, or feathery 
plants are usually very difficult to handle out of doors. The best plan is to 
photograph them in a room that is well and evenly lighted, or, best of all, 
in a stable, roomy tent. 

239. The sequence of details. No photographer ever escapes blunders 
entirely. At the outset of his work, the ecologist must fully realize this, 
and accordingly plan a method of operating the camera which will reduce 
the chance of mistake to a minimum. The usual blunders which every 
one makes sooner or later, such as making two exposures on one 
plate, drawing the slide before closing the shutter, allowing the light to 
strike the plate through the slit in the holder, etc., can be all but absolutely 
avoided by a fixed order of doing things. This order will naturally not be 
the same for different persons ; it is necessary merely that each have his own 
invariable sequence. The following one will serve as an illustration. As a 
preliminary, the plate-holders are filled, after having been carefully dusted, 
and the slides are uniformly replaced with the black edge inward. It is a 
wise precaution to again see that all the slides are in this position before 
leaving the. dark room. This will ensure that, a black edge outward always 
means that the plate has been exposed. The tripod is first set up and placed 
in what seems about the proper position. The camera is next attached to it, 
and the front and back opened. The bellows is pulled out, a short distance 
for views, and a longer one for detail pictures, and fastened. It is necessary 
to move the diaphragm index to the largest aperture and to open the shutter 
at "time." The next steps are to orient the view or object, and to bring it 
into sharp focus upon the ground glass. The first is accomplished by moving 
the entire instrument, changing the position of the tripod legs, swinging the 
camera upon the tripod, or by raising or lowering the lens front. It is often 
desirable also to change the position of the object on the plate by use of the 
reversible back. In views with much distance, the foreground is brought 
into sharp focus. In close views, especially of quadrats, the swing is used 
to increase the distance for the foreground, and the focus is made upon the 
center. After focusing, the shutter is closed, the indicator set at the time 



PHOTOGRAPHY I93 

desired, and the diaphragm "stopped down" as far as possible. Plate-holder 
1 is slipped into place, care being taken not to move the camera by a sudden 
jar. The camera cloth is dropped above the holder and allowed to hang 
down over the slide end. The slide is drawn and put on top of the instru- 
ment, the black edge always up. The exposure is made and the slide replaced 
with the black edge outzvard. This point should receive the most critical 
attention, as a blunder here will often cause the loss of two negatives. The 
plate-holder is returned to the receptacle, or merely placed in the back of 
the camera, which is then closed. The number of the plate, the name of the 
view or object, the condition of the light, the length of exposure, and the 
aperture of the diaphragm, as well as the date, are recorded in a notebook 
for this purpose. The shutter is then opened at "time," the diaphragm 
thrown wide open, and the front of the camera closed. When distances are 
short, the camera is often carried upon the tripod. As a rule, however, it is 
usually removed, and the tripod folded. In making subsequent pictures, 
the plates should always be used in their numerical order. 

240. The time of exposure is obviously the most critical task in the 
manipulation of a camera. The time necessary for a proper exposure varies 
with the season, the hour, the condition of the sky, the light intensity of 
the formation, the color and size of the area to be photographed, and, finally, 
of course, with the aperture of the diaphragm. Fortunately for the ecologist, 
the variation in light intensity during the season, and even during the 
greater part of the day, is not great, and can ordinarily be ignored. The 
beginner will make the most progress by determining the exposure de- 
manded by his instrument for taking a general view in full sunlight and with 
the smallest stop of the diaphragm. In standard cameras with lenses of 
ordinary rapidity, this is usually about one second. This will serve as a 
basis from which all other exposures may be reckoned until one has worked 
through a wide range of conditions and can recall just what time each view 
requires. On completely cloudy days the time required is five to ten times 
that necessary on a clear day ; filmy clouds and haze necessitate an exposure 
of two or three seconds. The more open forest formations demand an ex- 
posure of about five to ten seconds on a sunny clay, while the deeper ones 
require two or three times as long. A close view requires more time than 
a distant one, since the light-reflecting surface is much smaller. Quadrats 
require two or three seconds, and individual groups frequently take a longer 
time. The color of the vegetation plays an important part also : a dark green 
spruce forest requires twice as long an exposure as the aspen forest, and a 
grassland quadrat takes more time than one located in a gravel slide. In 
this connection, it is hardly necessary to point out that the lighted side of 



194 



THE FORMATION 



objects should always be taken, never the shaded one. The exposures in- 
dicated above are based upon the smallest stop. The reasons for using this 
whenever possible have already been given. When a larger stop is necessary, 
the exposure is decreased to correspond ; for example, a quadrat that takes 
three to four seconds at 256 can be taken at 64 in one second. As a rule, 
the sun should not be in front of the camera, but, when necessary, views can 
be made in this position if the sun is prevented from shining directly into 
the lens. 

241. Developing is as important as exposing. Indeed, it may well be 
considered more important, since a properly exposed plate may be spoiled in 
developing, while an under-exposure or over-exposure may be saved. 
Owing to the ease with which plants move in the wind, the ecologist is 
obliged to reconcile himself to many under-exposures, which can be con- 
verted into good negatives only by skilful developing. Every base station 
should have a good dark room, equipped with running water when possible, 
a good ruby lantern, and the proper trays and chemicals. Prepared develop- 
ing solutions are alluring because of their convenience, but after an extended 
trial of several kinds, the writer has reached the conviction that pyrogallic 
acid, or "pyro," is by far the most satisfactory in working with vegetation. 
Of almost innumerable formulae, the following gives excellent satisfaction 
and is convenient to use. 



I. 

500 cc. water 
30 grams sodium sulphite 
30 grams sodium carbonate 



II. 
500 cc. water 

5 grams pyrogallic acid 



For developing, equal parts of I and II are mixed, and a few drops of a 
10 per cent solution of potassium bromide added, unless there is reason 
to suspect that the plate has been seriously underexposed. The fixing 
bath is a concentrated solution of sodium hyposulphite, "hypo," to which 
a few drops of acetic acid are added. It should be replaced every week or 
two, depending upon how much it is used. A tray of water is kept at hand 
for bringing out the detail in underexposed negatives, and a second tray 
is used for washing. The "pyro" and the bromide solution should always 
be within reach, the former for accelerating, and the latter for retarding 
the development of unsatisfactory plates. 

The image will begin to show on a properly exposed plate within one 
to three minutes after it has been put in the developer. If the image 
appears almost instantly, and then recedes quickly, the plate is badly over- 



P PICTOGRAPHY IQ5 

exposed, and should be thrown away. In case it "comes up" less quickly, 
indicating that it is not greatly overexposed, it can be saved by the addition 
of more bromide. When the image does not show till the end of five to 
ten minutes, the plate has been underexposed. It is then necessary to add 
more "pyro," taking care not to pour it on the plate, and, after the image 
appears with its striking contrast, to leave the plate in water until as much 
detail as possible is brought out in the shadows. In the case of a normal 
exposure, when greater detail is desired, the negative is left for some time 
in water, and when contrast is sought more "pyro" is used. Negatives 
with unusual detail lack "snap" ; they are "flat," and fail to make artistic 
pictures. Contrast,' on the other hand, often obscures detail, and the best 
results can only be obtained by a happy combination of the two. The 
most important maxim in developing is that the process shall be con- 
tinued until the image has become indistinct. The universal tendency of 
the beginner is to remove the negative the moment the outlines grow 
dimmer, and the result is a thin, lifeless negative. It is almost impossible 
to develop too far, if the image is not allowed to disappear. Negatives of 
this sort are "thick," and though they print more slowly, produce brilliant 
pictures. A large quantity of the developing solution is used with single 
plates in small trays, and is allowed to act without rocking the tray. Much 
time is saved, however, by developing several plates together, and to avoid 
using a large quantity of the solution, the tray is gently rocked from time 
to time. This movement is particularly necessary at the beginning, in 
order that the plates may be covered evenly, and at once. Fifty cubic centi- 
meters of the solution will develop three or four 6y 2 x Sy 2 plates, and twice 
as many 4 x 5's. After the developer has once been used, it is kept for 
several days to restrain overexposed plates. As soon as the plate is 
developed, it is rinsed in water, and placed in the fixing fluid, until the 
white opaqueness is entirely removed. The "hypo" is then washed out 
by immersing the negatives for one to two hours in running water. If the 
latter can not be secured, the water in which they are placed should be 
changed frequently. The negatives are then air-dried within doors, in a 
place free from dust. Finally, they are filed away in negative envelopes, 
each bearing the name and number of the negative, and preferably also, 
the time and other exposure data. 

242. Finishing. On account of the time demanded by other field tasks, 
it has not been found desirable to make and finish prints in the field. This, • 
with the making of lantern slides, enlargements, etc., may well be turned 
over to a professional photographer. It is the custom to make a proof of 
each negative to meet the casual needs that arise in the field. For this 



I96 THE FORMATION 

purpose, solio "seconds" are used, since they are both cheap and satisfactory. 
When an urgent demand for a finished print does arise, it is met by using 
"velox" paper, which can be exposed in the dark room, and then developed 
and fixed exactly like a plate. Two standard papers for views are "solio" 
and "platina." The former gives brown tones, and is used for contrast and 
brilliancy, hence it is especially good for printing from negatives that have 
too much detail and too little contrast. "Platina," on the contrary, yields 
soft gray tones, and softens contrasts. 

FORMATION AND SUCCESSION HERBARIA 

243. Concept and purpose. A formation herbarium is a collection of 
exsiccati, in which the species are arranged with respect to their position 
in the formation, instead of being grouped in genera and families. Its 
primary purpose is to furnish a record of the constitution and the structure 
of a formation or a series of formations. At the same time, it affords the 
basal material for developing the subject of comparative phytogeography. 
It is impossible for one ecologist to visit many remote regions, to say noth- 
ing of spending a period sufficient for obtaining even a fair knowledge of 
the vegetation. He can at the best acquire an acquaintance with but few 
regions at first hand. In consequence, a method that brings a vegetation to 
him, with its structure carefully wrought out by years of study, is of the 
highest value. Time, as well as distance, sets a narrow limit to the number 
of formations which one man can investigate critically in a lifetime. It 
is no longer possible for a botanist to explore vast regions, and to bring 
back results which have anything more than a very general value. This 
fact, far from restricting the comparative study of vegetation, will serve 
to make it more accurate and systematic. The exact, results of numerous 
resident investigators, expressed in formation herbaria, with the proper 
series of quadrat maps and photographs, will be worked over by men who 
are themselves specially acquainted with a particular vegetation. Compari- 
sons will be founded upon a definite basis, and the relationship of various 
vegetations can then be expressed in precise rather than general terms. It 
is hardly too sweeping to assert that accurate work in the field of compara- 
tive phytogeography can be done only in this fashion. The value of forma- 
tion herbaria in class work is evident. On account of the limitations of 
time and distance, classes can touch but few formations, and these at every 
time except the growing period. For these reasons, an accurate and com- 
plete formational record that can be consulted or studied at any time is 
almost indispensable to class study in the development and structure of 
formations. 



FORMATION AND SUCCESSION HERBARIA I97 

244. Details of collecting. Formational collections, unlike the ordinary 
sets of exsiccati, can not be made upon the first visit to a region, or by 
a single journey through it. The determination of formation limits, and 
of developmental stages, of aspects, layers, abundance, etc., must necessarily 
precede, a work which alone takes several years. Moreover, collecting itself 
requires more than one year in a region containing numerous formations. 
This is exemplified by the Herbaria Formationum Color adensmm. 1 The 
preliminary study for this was made from 1896- 1899, the collecting was 
done chiefly in 1900 and 1901, while additional numbers were added in 
1902-3. For the purposes of the formation herbarium, specimens should 
be collected and pressed in such fashion as to show all the ecological fea- 
tures possible. Plants must be collected both in flower and in fruit, with 
the underground parts as perfect as may be. Seedlings and rosettes should 
be included whenever present.' In pressing, one or two leaves should be 
arranged with the lower side uppermost to admit of the ready comparison 
of both surfaces. Opened flowers are valuable for flower biology, while 
seeds and fruits are desirable for showing migration contrivances. The 
ferns, mosses, and lichens of the formation should be fully represented, to- 
gether with the more important fungi and algae. The number of photo- 
graphs taken for each herbarium should be limited only by considerations 
of time and expense. The ideal series consists of a general view of each 
formation, showing its physiographic setting, nearer views of each of its 
aspects, detail views of its consocies, societies, and layers, and flower portraits 
of all the constituent species. Such a series can only be obtained by resi- 
dence through a long term of years, and in most cases general and aspect 
views, with portraits of the facies and a few of the striking principal 
species, must suffice. Quadrat and transect charts, together with forma- 
tional maps, are extremely desirable, and, indeed, all but indispensable. 

245. Arrangement. The arrangement of species within each formation 
herbarium is based upon the structure of the vegetation. The primary 
groupings are made with reference to time of appearance and abundance; 
when definite zones, associations, or layers are present, they must likewise 
be taken into account. In the Colorado collection, the first division is into 
three aspects based upon the period of flowering (aspectus vemalis, aesti- 
valis, autumnalis). Within each aspect, the species are arranged with re- 
spect to abundance in the groups, facies, principal species, and secondary 
species. Each group is placed in an ordinary manila cover, which bears a 
printed label indicating the aspect and the group. The species labels give, 

1 Clements, F. E. and E. S. Herbaria Formationum Coloradensium. 1902. 



19^ THE FORMATION 

in addition to the name, date, and place of collection, the phyad or vegeta- 
tion form, the geographical area, the rank of the species, the aspect, and 
the formation. To these may well be added data concerning migration 
contrivances, seed production, pollination, period of flowering, etc. The 
photographs are mounted on the usual herbarium sheets, and placed in the 
proper order in the various groups, and a similar disposition is made of 
quadrat and transect charts, and such physical factor summaries as seem 
desirable. 

246. Succession herbaria. The arrangement of formation herbaria may 
follow the classification of formations with respect to character, region, or 
development. The first is the most convenient for purposes of instruction, 
and has distinct advantages in permitting a close comparison of the vegeta- 
tion of different habitats. The second basis, which is the one used in the 
Herbaria Formationum Color ad ensium, is peculiarly adapted to mountain 
vegetation in which the zones are usually very distinct. The arrangement 
of herbaria in a developmental series, however, is the most logical and the 
most illuminating, since the structure of the ultimate formations is not only 
made plain, but the stages in their development are also laid bare. Such 
succession herbaria are the natural outgrowth of formational ones. Indeed, 
the latter should be made merely the starting point for these in all regions 
where the causes which bring about successions are active. Where weather- 
ing is still an important factor, as in mountains, the initial and intermediate 
formations which lead to the final grassland or forest are often in evidence. 
After a formation herbarium of each stage has been made in the way in- 
dicated, a succession herbarium is obtained merely by arranging the various 
herbaria in the sequence of the developmental stages. Thus, in the Colo- 
rado collection, the subalpine formations are arranged according to altitude 
in the following series: (1) the pine formation, (2) the gravel slide for- 
mation, (3) the half gravel slide formation, (4) the aspen formation, 
(5) the balsam-spruce formation, (6) the spruce-pine formation, (7) the 
meadow thicket formation, (8) the brook bank formation. Of these, five 
belong to the same succession, and it is possible to indicate the development 
of the spruce-pine forest by arranging these five formations in their proper 
order in a succession herbarium, as follows : ( 1 ) the gravel slide formation, 
(2) the half gravel slide formation, (3) the pine formation, (4) the bal- 
sam-spruce formation, (5) the spruce-pine formation. 



development and structure 199 

Development and Structure 

247. Vegetation an organism. The plant formation is an organic unit. 
It exhibits activities or changes which result in development, structure, and 
reproduction. These changes are progressive, or periodic, and, in some 
degree, rhythmic, and there can be no objection to regarding them as 
functions of vegetation. According to this point of view, the formation is 
a complex organism, which possesses functions and structure, and passes 
through a cycle of development similar to that of the plant. This concept 
may seem strange at first, owing to the fact that the common understanding 
of function and structure is based upon the individual plant alone. Since 
the formation, like the plant, is subject to changes caused by the habitat, 
and since these changes are recorded in its structure, it is evident that 
the terms, function and structure, are as applicable to the one as to the 
other. It is merely necessary to bear in mind that the functions of plants 
and of formations are absolutely different activities, which have no more 
in common than do the two structures, leaf and zone. 

248. Vegetation essentially dynamic. As an organism, the formation 
is undergoing constant change. Constructive or destructive forces are 
necessarily at work; the former, as in the plant, predominate until maturity, 
when the latter prevail. Consequently, it no longer seems fruitful to classify 
the phenomena of vegetation as dynamic or static. The emphasis which 
lias been placed upon dynamic aspects of vegetation has served a useful 
purpose by calling attention to the development of the latter. Although it 
is a quarter of a century since Hult, and more than a half century since 
Steenstrup, by far the greater number of ecological studies still ignore the 
problem of development. This condition, however, can be remedied more 
easily by insisting upon an exact understanding of the nature of the forma- 
tion than in any other way. It is entirely superfluous to speak of dynamic 
and static effects in the plant, and the use of these terms with reference 
to the formation becomes equally unnecessary as soon as the latter is 
looked upon as an organism. The proper investigation of a formation can 
no more overlook development than structure, so closely are the two inter- 
woven. Future research must rest squarely upon this fact. 

249. Functions and structures. The functions of a formation are as- 
sociation, invasion, and succession : the second may be resolved into migra- 
tion and ecesis, and the third, perhaps, into reaction and competition. 
Formational structures comprise zones, layers, consocies, societies, etc., all 
of which may be referred to zonation, or to alternation. The term associa- 



200 THE FORMATION 

tion has been used in both an active and a passive sense. In the former, it 
applies to the inevitable grouping together of plants, by means of reproduc- 
tion and immobility. Passively, it refers to the actual groupings which 
result in this way, and in this sense it is practically synonymous with vege- 
tation. Invasion is the function of movement, and of occupying or taking 
possession ; with association, it constitutes the two fundamental activities 
of vegetation. It is the essential part of succession, but the latter is so 
distinctive, because of the intimate relation of competition and re- 
action, that clearness is gained by treating it as a separate function 
which is especially concerned with development. Association, zonation, 
and alternation are structural phenomena, which are in large part the 
immediate product of habitat and function, and in a considerable degree, 
also, the result of ancestral or historical facts. It is a difficult matter to 
determine in what measure the last factor enters, but it is one that must 
always be taken into account, particularly when the physical factors of 
the habitat are inadequate to explain the structures observed. Structurally, 
association reo'ularlv includes both zonation and alternation. As there are 
certain typical instances in which it exhibits neither, the treatment will be 
clearer if each is considered separately. 

ASSOCIATION 

250. Concept. The principle of association is the fundamental law of 
vegetation. Indeed, association is vegetation, for the individual passes into 
vegetation, strictly speaking, at the moment when other individuals of the 
same kind or of different kinds become grouped with it. It is then (and 
the same statement necessarily holds for vegetation) the coming together 
and the staying together of individuals and, ultimately, of species. A con- 
crete instance will illustrate this fact. In the development of the blowout 
formation of the Nebraska sand-hills (Rcdfieldia-Muhlenbergia-anem- 
ium)$ association begins only when the first plant of Redfieldia ftexuosa 
is joined by other plants that have sprung from it, or have wandered in over 
the margin of the blowout. Henceforth, whatever changes the blowout for- 
mation may undergo, association is a settled characteristic of it until some 
new and overwhelming physical catastrophe shall destroy the associated 
individuals. It will readily be seen that association does not depend upon 
particular individuals, for these, pass and others take their place, but that 
it does depend essentially upon number of individuals. 

Association involves the idea of the relation of plants to the soil, as well 
as that of plants to each other. It is synonymous with vegetation only 
when the two relations are represented, since there may be association such 



ASSOCIATION 201 

as that of a parasite with its host, which does not constitute vegetation. 
But it will be seen that the relation of the parasite to the host is practically 
identical with the relation of the plant to the soil or stratum, and the two 
concepts mentioned above become merged in such a case. From this it 
follows that association results in vegetation only when the two ideas are 
distinct. The concept of association contains a fact that is everywhere 
significant of vegetation, namely, the likeness or unlikeness of the individ- 
uals which are associated. In the case of parasite and host, this unlikeness 
is marked ; in vegetation, all degrees of similarity obtain. As will be evi- 
dent when the causes which lead to association are considered, alternate 
similarity and dissimilarity of the ccnstituent individuals or species is subor- 
dinate as a feature of vegetation only to the primary fact of association. 

Since association contains two distinct, though related, ideas, it is of 
necessity ambiguous. It is very desirable that this be avoided, in order that 
each concept may be clearly delimited. For this reason, the act or process 
of grouping individuals is termed aggregation, while the word association 
is restricted to the condition or state of being grouped together. In a 
word, aggregation is functional, association is structural; the one is the 
result of the other. This distinction makes clear the difference between 
association in the active and passive sense, and falls in with the need of 
keeping function and structure in the foreground. 

251. Causes. In considering the causes which produce association, it 
is necessary to call m evidence the primary facts of the process in concrete 
examples of this principle. These facts are so bound up in the nature of 
vegetal organisms that they are the veriest axioms. Reproduction gives 
rise immediately to potential, and ultimately, in the great majority of cases, 
to actual association. The degree and permanence of the association are 
then determined by the immobility of the individuals as expressed in terms 
of attachment to each other or to the stratum, such as sheath, thallus, 
haustoria, holdfasts, rhizoids, roots, etc. The range of immobility is very 
great. In terrestrial plants, mobility is confined almost entirely to the 
period when the individual lies dormant in the seed, spore, or propagative 
part, which is alone mobile. In aquatic spermatophytes, the same is true 
of all attached forms, while free floating plants such as Lcmna are mobile 
in a high degree, especially during the vegetative period. Among the 
algae and hydrophilous fungi, attached forms are mobile only in the spore 
or propagative condition, while the motile forms of the plancton typify the 
extreme development of mobility. The immediate result of reproduction 
in an immobile species is to produce association of like individuals, while in 
the case of a mobile species reproduction may or may not lead immediately 



202 THE FORMATION 

to association. We may lay down the general principle that immobility tends 
to maintain the association of the individuals of the same generation, i. e., 
the association of like forms, while mobility tends to separate the similar 
individuals of one generation and to bring unlike forms together. With 
the mobile algae, separation of the members of each generation is the rule, 
unless the individuals come to be associated in a thallus, or are grouped in 
contact with the substratum. Flowering plants that are relatively immobile, 
especially in the seed state, drop their seeds beneath and about the parent 
plants, and in consequence dense association of the new plants is the rule. 
In very many cases, however, this primitive tendency is largely or completely 
negatived by the presence of special dissemination contrivances, which are 
nearly, if not quite, as effective for many terrestrial plants as the free float- 
ing habit is for algae. From this point, the whole question of mobility be- 
longs to migration, just as the adjustment between the parent plants and 
their offspring, or between plants established and the mobile plants to be 
established, belongs to competition. 

If association were determined by reproduction and immobility alone, it 
would exhibit areas dissimilar in the mass of individuals, as well as areas 
dissimilar in the kinds of individuals. Some areas would be occupied by 
plants of a single species, others by plants of several or many species. This 
tendency of association to show differences is, however, greatly emphasized 
by the fact that vegetation is fundamentally attached to and dependent upon 
a surface that exhibits the most extreme physical differences. For this rea- 
son, new differences in association appear, due not only to the morphological 
differentiation of vegetation forms, but also to the changes in the degree and 
manner of association produced directly by the different habitats. Associa- 
tion might then be defined as a grouping together of plant individuals, of 
parents and progeny, which is initiated by reproduction and immobility, and 
determined by environment. It is a resultant of differences and similarities. 
In consequence, association in its largest expression, vegetation, is essen- 
tially heterogeneous, while in those areas which possess physical or biolog- 
ical definiteness, habitats and vegetation centers, it is relatively homogene- 
ous. This fundamental peculiarity has given us the concept of the 
formation, an area of vegetation, or a particular association, which is homo- 
geneous within itself, and at the same time essentially different from con- 
tiguous areas, though falling into a phylogenetic series with some and a 
biological series with others. From its nature, the plant formation is to be 
considered the logical unit of vegetation, though it is not, of course, the 
simplest example of association. 



ASSOCIATION 20"} 

252. Aggregation. As indicated under the causes of association, the 
process by which groups of individuals are formed depends entirely upon 
reproduction and migration. In short, aggregation is merely a corollary of 
movement. The simplest example of this process occurs in forms like Gloe- 
ocapsa, Tetraspora, and others, where the plants resulting from fission are 
held together by means of a sheath. Though called a colony, such a group 
of individuals is a family in the ordinary sense. Practically the same group- 
ing results in the case of terrestrial plants, especially spermatophytes, when 
the seeds of a plant mature and fall to the ground about it. The relation in 
both instances is essentially that of parent and offspring, although the parent 
soon disappears in the case of annuals, while among the algae its existence 
is regularly terminated by fission. The size and the density of the family 
group are determined by the number of seeds produced, and by their mo- 
bility. These are further affected by the height and branching of the plant, 
and by the position of the seeds upon it. The disseminules of immobile 
species fall directly beneath the parent, and the resulting group is both uni- 
form and definite. A similar arrangement is caused likewise by offshoots. 
An increase in mobility brings about a decrease of aggregation, since the 
disseminules are carried away from the parent plant. Perfectly mobile 
forms rarely produce family groups for this reason. It is evident, however, 
that mobile perennials sometimes arrange themselves in similar fashion in 
consequence of propagation by underground parts. Consequently, it is pos- 
sible to state the law of single aggregation, viz., that immobility promotes 
the grouping of parent and offspring, and mobility hinders it. 

If all species were immobile, the family group would be characteristic of 
vegetation. Since the great majority are more or less mobile, aggregates 
of this sort are the exception rather than the rule. Mobility not only de- 
creases the number of offspring in the family group, but it also spreads dis- 
seminules broadcast to enter dissimilar groups. It leads directly to mixed 
aggregation, by which individuals of one or more species invade the family 
group. Once established, the newcomers tend also to produce simple groups, 
thus causing an arrangement corresponding essentially to a community. 
Such collections of family groups are extremely variable in size and defini- 
tion. This arises in part from the nature of simple aggregation, and in part 
from the varying mobility of different species. Mobility alone often pro- 
duces similar communities by bringing together the disseminules of different 
plants, each of which then becomes the center of a mixed group. In the 
case of permobile species, several disseminules of each may be brought to- 
gether. The resulting area, though larger, is practically the same. At pres- 
ent, it is difficult to formulate the law for this method of grouping. It may 
be stated provisionally as follows : mixed aggregation is the direct result of 
mobility, and the greater the mobility the more heterogeneous the mixture. 



204 THE FORMATION 

The constitution of all the major areas of a formation is to be explained 
upon the basis of aggregation by the two methods described. The relative 
importance of family groups and communities differs for every formation, 
and the exact procedure in each can be obtained only by the detailed study 
of quadrats. The problem is further complicated by competition and reac- 
tion, particularly in closed vegetation. For this reason, aggregation can be 
studied most satisfactorily in a new or denuded area, where these processes 
are not yet in evidence. 

Kinds of Association 

253. Categories. In the analysis of association, it must be kept clearly in 
mind that the concrete examples from which all generalizations must be 
drawn are often in very different stages of development, and are of corre- 
spondingly different ages. For this reason it has seemed best to consider 
the primary relations of association in general in this place, leaving the 
treatment of the effects of invasion, succession, alternation, and zonation to 
he taken up under these topics. 

Various categories of association may be distinguished, according to the 
dominant physical factor concerned or the point of view taken. These will 
fall into two series, as we consider the relation of plant to plant with refer- 
ence to some object or characteristic, or the grouping of plants together in 
response to some dominant factor. In the first series may be placed asso- 
ciation with reference to substratum, to the ground (occupation), and to 
invasion ; in the second belong light and water-content association. It 
should be noted that these are all actual associations in nature, and not con- 
cepts such as the vegetation form, within which plants from widely different 
associations may be classified. Naturally, it does not follow that it is not 
logical or valuable to group together those plants, such as hydrophytes, 
sciophytes, hysterophytes, etc., which have a common relation to some factor, 
but belong to different formations. 

254. Stratum association. Plants manifest independent or dependent as- 
sociation with reference to the stratum to which they are attached and from 
which they derive food or support. Independent association is exhibited by 
those hoiophytic species of a formation w r hich are entirely independent of 
each other with respect to mechanical support or nutrition. It is charac- 
teristic of the greater number of the constituent species, of formations. De- 
pendent association is manifested in the relation between host and parasite, 
stratum and epiphyte, support and liane. Warming 1 has distinguished six 

1 Lehrbuch der Okologischen Pflanzengeographie, 97. 1896. 



ASSOCIATION 205 

kinds of associations : parasitism, helotism, mutualism, epiphytism, lianism, 
and commensalism. Coramensalism corresponds to the primary principle 
of association which has given rise to vegetation. Homogeneous commen- 
salism is the term applied to social exclusive plants, in which the patch is 
composed of a single species. Such association is extremely rare in nature, 
and if the most minute forms be considered, probably never occurs. On the 
other hand, heterogeneous commensalism, in which individuals of more than 
one species are present, is everywhere typical of vegetation. Warming re- 
gards saprophytism merely as a specialized kind of parasitism, an opinion 
that may well be defended. Helotism, however, is also a mere modification 
of parasitism, if it is not indeed parasitism pure and simple. Mutualism is 
an altogether vague concept, including parasites, epiphytes, and endophytes 
of doubtful physiological relation. Pound and Clements 1 treated lianes, par- 
asites, and saprophytes as vegetation forms, relating herbaceous creepers 
and twiners to the lianes, and dividing the fungi and lichens into nine 
groups. Whatever the value of these divisions may be from the standpoint 
of vegetation forms, they represent the same relation between plant and nu- 
tritive stratum, and with respect to association should be merged in one 
group. Schimper 2 was the first to perceive the essential similarity of all 
such groups from the standpoint of association. He terms these plant so- 
cieties (Genossenschaften) , retaining the four groups already established, 
lianae, epiphyta, saprophyta, and parasiticae. It is evident that dependent 
association comprises extremely divergent forms, from the slightly clinging 
herb, such as Galium, to the most intense parasite. The distinction, how- 
ever, is a clear one, if restricted to that relation between plants in which one 
acts as a mechanical support or stratum or as a nutritive host for the other. 

255. Ground association. The first division of formations into open and 
closed was made by Engler and Drude. 3 Open formations were defined as 
those having incomplete stability and heterogeneous composition, while 
closed formations have a more definite uniform stamp. W 'hat is true of for- 
mations is equally true of vegetation, so that association may be regarded 
as open or closed with reference to the density and thoroughness with which 
the plants occupy the ground. In open association, the ground is slightly 
or partially occupied, readily permitting the entrance of new plants without 
the displacement of those already present. Such an arrangement is char- 
acteristic of the early stages of a formation, or of a succession of forma- 
tions. It produces unstable open formations, which arise, usually 

1 Phytogeography of Nebraska, 1st ed., 101. 1898. 

2 Pnamengeographie anf physiologischer Grundlage, 208. 1S98. 
8 Die Vegetation der Erde. Engler Bot. Jahrb,, 17 :b55. 1893. 



206 THE FORMATION 

after denudation, in sand-hills, blowouts, gravel slides, dunes, flood 
plains, burned areas, etc. In closed association, occupation of the ground is 
complete, and the invasion of new species can occur only through displace- 
ment. Closed association results in stable, closed formations, such as forest, 
thicket, meadow, and prairie. As open association characterizes the early 
stages of a succession of formations, so closed association is peculiar to the 
later or last stages of all such successions. In short, open formations rep- 
resent certain phases of the development of vegetation, while closed forma- 
tions correspond to the relatively final structural conditions. It is a funda- 
mental principle of association that every succession from denudation, or 
from newly formed soils, begins with open formations and ends with a 
closed formation. The causes leading up to open and closed association are 
intimately connected with development, and hence are considered under in- 
vasion and succession. 

256. Species guild association. Drude has distinguished a kind of asso- 
ciation peculiar to invasion, in which there is a successive or concomitant 
movement of certain species of a formation into another formation or region, 
resulting in species guilds (Artengenossenschaften). The association in this 
case is largely one of community of origin or area, and of concomitant mi- 
gration. It is especially characteristic of areas adjacent to formational and 
regional limits. Fundamentally, it is merely the grouping of plants which 
are invading at the same time, and consequently it differs only in degree 
from what occurs in every invasion w T here more than a single individual is 
concerned. Accordingly, this type of association has little more than his- 
torical interest. This must not be construed to mean that it does not occur, 
but that it differs in no essential from the ordinary grouping of invaders. 

257. Light association. The constituent species of formations show two 
fundamentally different groupings with respect to light. In the one case, 
the individuals are on the same level, or nearly so, in such a way that each 
has direct access to sunlight. Such an arrangement is characteristic of most 
grassland and herbaceous formations. In the case of desert formations, 
there is 'often considerable difference in the height of the plants, but the dis- 
tance between them is so great as to admit of direct illumination of all. 
This arrangement may be termed coordinate association. In forests, thick- 
ets, and many herbaceous wastes, the height and density of certain species 
enable them to dominate the formation. In a dense forest, the trees receive 
practically all the light incident upon the formation, and the shrubs, herbs, 
fungi, and algae of lower habit and inferior position must adapt themselves 
to the diffuse light which passes through or between the leaves. The same 



ASSOCIATION 207 

is equally true of dense thickets and wastes, except that the vertical distance 
is less, and the diffuseness of the light is correspondingly modified. In these 
formations, the dominant trees, shrubs, or herbs, the facies, constitute a pri- 
mary or superior layer. The degree of subordinate association, as a result 
of which inferior layers will arise, is entirely determined by the density of 
the facies. In open woodlands, which are really mixed formations of wood- 
land and grassland, the intervals, and usually the spaces beneath the trees 
also, are covered with poophytes, showing an absence of subordination due 
to light. This is the prevailing condition in the pine formation (Finns pon- 
der csa-xerohy Hum) of the ridges and foot-hills of western Nebraska. 
When, however, the trees stand sufficiently close that their shadows meet or 
overlap throughout the day, the increasing diffuseness begins to cause modi- 
fication and rearrangement of the individuals. By photometric methods, the 
light in a forest is found to be least diffuse just below the facies, while the 
diffuseness increases markedly in passing to the ground. The taller, 
stronger individuals are consequently in a position to assimilate more vig- 
orously, and to become still taller and stronger as a result. Just as these 
have taken up a position inferior to that of the facies, so the shorter or 
weaker species must come to occupy a still more subordinate position. This 
results, not only because the light is primarily weaker nearer the ground, 
but also because the taller. plants interpose as a second screen. The complete 
working out of this arrangement with reference to light produces typical 
subordinate association, which finds its characteristic expression in the lay- 
ering of forests and thickets. Layers tend to appear as soon as open wood- 
land or thicket begins to pass into denser conditions, and up to a certain 
point, at which they disappear, they become the more numerous and the 
more marked, the denser the forest. 

In the Otowanie woods near Lincoln (Qucrcus-Hicoria-hylinm) , layering 
usually begins at a light value of .1 (i=normal sunshine in the open). 
Thornber 1 has found the same value to obtain in the thickets of the Missouri 
bluffs. In these, again, layers disappear at a value of .005, the extreme 
diffuseness making assimilation impossible except for occasional mosses 
and algae. A number of herbaceous plants are present in the spring, but 
these are all prevernal or vernal bloomers, which are safely past flowering 
before shade conditions become extreme. In the Fraxinus-Catalpa-ahiiim, 
all inferior holophytic vegetation disappears between the light value of .004 
and that of .003. The spruce-pine formation (Picea-Pinus-hylium) of the 
Rocky mountains, with a light value of .01, usually contains but a few scat- 
tered herbs, mostly evergreen ; in some cases there are no subordinate plants 

1 Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 
5 :36, 46. 1901. 



208 THE FORMATION 

other than mosses and hysterophytes. The lodge-pole pine formation (Pinus 
murrayana-hylmm), with light values often less than .005, is nearly or quite 
destitute of all but hysterophytic undergrowth. Such extremely dense for- 
mations are examples of coordinate association merely, since the formation 
is reduced to a single superior layer, in which the individuals of the facies 
bear the same spatial relation to incident light. In layered formations, in 
addition to the subordinate relation of other species to the facies, there is, of 
course, a kind of coordinate association manifested in each layer. 

258. Water=content association. Schouw 1 was the first to give definite 
expression to the value of the water-content of the soil for the grouping of 
plants. He established four groups: (1) water plants, (2) swamp plants, 
(3) plants of moist meadows, (4) plants of dry soils. The first he termed 
hydrophytes, introducing the term halophytes to include all saline plants. 
Thurmann 2 recognized the fundamental influence of water-content upon as- 
sociation, and further perceived that the amount of water present was deter- 
mined primarily by the physical nature of the soil. He distinguished plants 
which grow in soils that retain water as hygrophilous, and those found upon 
soils that lose water readily as xerophUous. Those which seemed to grow 
indifferently upon either were termed ubiquitous. The latter correspond in 
some measure to mesophytes, but they are really plants possessing a con- 
siderable range of adaptability, and do not properly constitute a natural 
group. Warming 3 proposed the term mesophytes to include all the plants 
intermediate between hydrophytes and xerophytes. He recognized the para- 
mount value of water-content association as the basis of ecology, and upon 
this made a logical and systematic treatise out of the scattered results of 
many workers. Schimper 4 placed the study of vegetation upon a new basis 
by drawing a distinction between physical and physiological water-content, 
and by pointing out that the last alone is to be taken into account in the 
study of plant life, and hence of plant geography. Accepting the easily 
demonstrable fact that an excess of salts in the soil water, as well as cold, 
tends greatly to diminish the available water of the soil, i. e., the chresard, 
it is at once seen why saline and arctic plants are as truly xerophytic as those 
that grow on rocks or in desert sands. An anomalous case which, however, 
physical factor records have explained fully, is presented by many plants 
growing in alpine gravel slides, strands, blowouts, sandbars, etc., in which 
the water-content is considerable, but the water loss excessive, on account 

1 Grnndziige einer allgemeinen Pflanzengeographie, 157. 1823. 
2 Essai de phytostatique, etc. 1849. 
3 /. c mJ 116. 1896. 
4 /. c. 3. 1898. 



ASSOCIATION 209 

of extreme heat or reduced air pressure. The effect of these conditions is 
to produce a plant xerophytic as to its aerial parts, and mesophytic or even 
hydrophytic as to subterranean parts. Such plants may, from their twofold 
nature, be termed dissophytes ; they are especially characteristic of dysgeo- 
genous soils in alpine regions where transpiration reaches a maximum, but 
are doubtless to be found in all gravel and sand habitats with high water- 
content. With these corrections, the concept of water-content association, 
which owes much to both Warming and Schimper, but is largely to be cred- 
ited to Thurmann, becomes completely and fundamentally applicable to all 
vegetation. 

Up to the present time, the general character of the habitat, together with 
the gross appearance of the plant itself, has been thought sufficient to deter- 
mine the proper position of a plant or a formation in the water-content clas- 
sification. Such a method is adequate, however, only for plants and forma- 
tions which bear a distinct impress. For an accurate classification into the 
three categories, hydrophytes, mesophytes, and xerophytes, it is necessary 
to make exact determinations of the normal holard and chresard of the hab- 
itat, and to supplement this, in some degree at least, by histological studies. 
Except in the case of saline, acid, and frozen soils, the holard alone will be a 
fairly accurate index, especially in habitats of similar soil composition. For 
an exact and comprehensive classification, however, and particularly in com- 
parative work, the chresard must constitute the sole criterion. As the latter 
has been ascertained for very few formations, and in Nebraska and Colorado 
alone, the present characterization of many plants and formations as hydro- 
phytic, mesophytic, or xerophytic must be regarded as largely tentative, and 
the final classification will be possible only after the thorough quantitative 
investigation of their habitats. 

The water-content groups, hydrophytia, mesophytia, and xerophytia, in- 
clude all formations found upon the globe. The exactness with which this 
classification applies to vegetation is made somewhat more evident by divid- 
ing mesophytia into forest and grassland. This is based primarily upon light 
association, but it also reflects water-content differences in a large degree. 
The groups thus constituted represent the fundamental zonation of the veg- 
etative covering with respect to water-content. Ocean, forest, grassland, 
and desert correspond exactly to hydrophytia, hylophytia, poophytia, and 
xerophytia. The difference is merely one of terminology : the first series 
takes into account the physiognomy of the vegetation itself, while the other 
emphasizes the causative factors. 



210 THE FORMATION 

THE DEVELOPMENT OF THE FORMATION 

259. A strict account of development should trace the results of the vari- 
ous activities of vegetation in their proper sequence. This is aggregation, 
migration, ecesis, reaction, and competition. These functions are so inti- 
mately and often so inextricably associated that it is hardly feasible to dis- 
cuss development by treating each one separately. In consequence, the two 
fundamental phenomena, invasion and succession, which they produce, are 
taken as the basis of the discussion. These, moreover, are different only in 
degree ; succession is merely complete, periodic invasion. Nevertheless, the 
subject gains in clearness by a separate treatment of each. 

INVASION 

260. By invasion is understood the movement of plants from an area of 
a certain character into one of a different character, and their colonization 
in the latter. This movement may concern an individual, a species, or a 
group of species. From the nature of invasion, which contains the double 
idea of going* into and taking possession of, it usually operates between con- 
tiguous formations, but it also takes place between formational zones and 
patches. More rarely and less noticeably, there may be invasion into a re- 
mote vegetation, as a result of long carriage by wind, water, birds, railroads, 
or vessels. Movement or migration, however, represents but one of the two 
ideas involved in invasion. Migration merely carries the spore, seed, or 
propagule into the area to be invaded. In ecesis, the spores or seeds germi- 
nate and grow, after more or less adjustment, and in case the latter becomes 
sufficiently complete, the new plants reproduce and finally become estab- 
lished. With all terrestrial plants, invasion is possible only when migration 
is followed by ecesis, because of the inherent differences of formations or of 
areas of the same formation. In the case of surface floating forms, such as 
Lcmnaceae, and of the plancton, ecesis is of much less importance, on ac- 
count of the uniformity of the medium and the lack of attachment, and 
migration is often practically synonymous with invasion. 

MIGRA HON 

261. Migration has been sometimes used loosely as a synonym for inva- 
sion, but it is here employed in its proper sense of removal or departure, i. e., 
movement, and is contrasted with ecesis, the making of a home, the two 
ideas being combined in invasion, which is a moving into and a taking pos- 
session of. An analysis of migration reveals the presence of four factors, 
mobility, agency, proximity, and topography. Not all of these are present 



INVASION 211 

in every instance of migration, as for example in the simple elongation of a 
rootstalk, but in the great majority of cases each plays its proper part. Mo- 
bility represents the inherent capacity of a plant for migration, and in its 
highest expression, motility, is in itself productive of movement. As a gen- 
eral rule, however, modifications for securing mobility are ineffective in the 
absence of proper agents, and the effective operation of the two will be pro- 
foundly influenced by distance and topography. 

262. Mobility denotes potentiality of migration as represented by modi- 
fications for this purpose. It corresponds, in a sense, to dissemination, 
though seed production also enters into it. Its most perfect expression is 
found in those plants which are themselves motile, Bacteriaceae, Oscillatoria, 
Volvocaceae, and Bacillariaceae, or possess motile propagules, such as most 
Phycophyta. On the other hand, it is entirely undeveloped in many plants 
with heavy unspecialized seeds and fruits. Between these two extremes lie 
by far the greater number of plants, exhibiting the most various degrees of 
mobility, from the motile , though almost immobile offshoots of many Lili- 
aceae to the immotile but very mobile spores of fungi. It is thus seen that 
motility plays a relatively small part in migration, being practically absent 
in terrestrial forms, and that it bears a very uncertain relation to mobility. 
In analyzing the latter, contrivances for dissemination are seen to determine 
primarily the degree of mobility, while the number of seeds produced will 
have an important effect in increasing or decreasing it. A third factor of 
considerable importance is also involved, namely, position with reference to 
the distributive agent, but any exact knowledge of its importance must await 
systematic experiment somewhat after the methods of Dingier, but with air- 
currents, etc., of known velocity and direction. The time is not distant when 
by such methods it will be possible to establish a coefficient of mobility, de- 
rived from terms of position, weight, resistant surface, and trajectory for 
definite wind velocities or for particular propulsive mechanisms. 

263. Organs for dissemination. Plants exhibit considerable diversity 
with reference to the part or organ modified, or at least utilized, for dissem- 
ination. This modification, though usually affecting the particular product 
of reproduction, may, in fact, operate on any part of the plant, and in certain 
cases upon the entire plant itself. In the majority of plants characterized 
by alternation of generations, the same individual may be disseminated in 
one generation by a reproductive body, and in the other by a propagative 
one, as is the case in the oogones and conidia of Peronospora, the spores and 
gemmae of Marchaniia, the fruits and runners of Fragaria, etc. Special 
modifications have, as a rule, been developed in direct connection with spores 



212 THE FORMATION 

and seeds, and mobility reaches its highest expression in these. It is, on the 
other hand, greatly restricted in offshoots and plant bodies, at least in ter- 
restrial forms, though it will now and then attain a marked development in 
these, as shown by the rosettes of Sempcrviviim and the tumbling plants of 
Cycloloma, For the sake of convenience, in analyzing migration, all plants 
may be arranged in the following groups with reference to the organ or part 
distributed. 

i. Spore-distributed, sporostrotes. This includes all plants possessing 
structures which go by the name of spore, such as the acinetes of Nostoc 
and Protococctis, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia, 
ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and 
the gemmae and spores proper of liverworts, mosses, and ferns. These are 
almost always without especial contrivances for dissemination, but their ex- 
treme minuteness results in great mobility. 

2. Seed-distributed, spermatostrotes. This group comprises all flowering 
plants in which the seed is the part modified or at least disseminated. The 
mobility of seeds is relatively small, except in the case of minute, winged or 
comate seeds. 

3. Fruit-distributed, carpostrotes. The modifications of the fruit for dis- 
tribution exceed in number and variety all other modifications of this sort. 
All achenes, perigynia, utricles, etc., properly belong here. 

4. Offshoot-distributed, thallostrotes. To this class are referred those 
plants, almost exclusively cormophytes, which produce lateral, branch-like 
propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc. 
Migration with such plants is extremely slow, but correspondingly effective, 
since it is almost invariably followed by ecesis. 

5. Plant-distributed, phytostrotes. This group includes all plancton and 
surface forms, whether motile or non-motile, and those terrestrial plants in 
which the whole plant, or at least the aerial part, is distributed, as in tumble- 
weeds and in many grasses. 

264. Contrivances for dissemination. Any investigation of migration to 
be exact must confine itself to fixed forms. For these the degree of perfec- 
tion shown by dissemination contrivances corresponds almost exactly to the 
degree of mobility. Because of the difficulty of ascertaining the effect of 
ecesis, it is impossible to determine the actual effectiveness in nature of dif- 
ferent modifications, and the best that can be done at present is to> regard 
mobility, together with the occurrence and forcefulness of distributive agents, 
as an approximate measure of migration. The general accuracy of such a 
measure will be more or less evident from the following. Of 118 species 
common to the foot-hill and sand-hill regions of Nebraska, regions which are 



INVASION 213 

sufficiently diverse to indicate that these common species must have entered 
either one by migration from the other, 83 exhibit modifications for dissem- 
ination, while 8 others, though without special contrivances, are readily dis- 
tributed by water, and 4 more are mobile because of minuteness of spore or 
seed. Some degree of mobility is present in 73 per cent of the species com- 
mon to these regions, while of the total number of species in which the mode 
of migration is evident, viz., 95, 66 per cent are wind-distributed, 20 per cent 
animal-distributed, and 14 per cent are water-distributed. It need hardly be 
noted that this accords fully with the prevalence and forcefulness of winds 
in these regions. Of the species peculiar to the foot-hill region, many are 
doubtless indigenous, though a majority have come from the montane regions 
to the westward. The number of mobile species is 121, or 60 per cent of the 
entire number, while the number of wind-distributed ones is 85, or 70 per 
cent of those that are mobile. Among the 25 species found in the widely 
separated wooded bluff and foot-hill regions, 2 only, Amorpha nana and 
Roripa nasturtium, are relatively immobile, but the minute seeds of the lat- 
ter, however, are readily distributed, and the former is altogether infrequent. 
The following groups of plants may be distinguished according to the 
character of the contrivance by which dissemination is secured : 

1. Saccate, saccospores. Here are to be placed a variety of fruits, all of 
which agree, however, in having a membranous envelope or an impervious, 
air-containing pericarp. In Ostrya, Pliysalis, Staphylea, the modification 
is for wind-distribution, while in Carex, Nymphaea, etc., it is for water- 
transport. 

2. Winged, pterospores. This group includes all winged, margined, and 
flattened fruits and seeds, such as are found in Acer, Betula, Rumex, many 
Umbellifcrae, Graminaceac, etc. 

3. Comate, comospores. To this group belong those fruits and seeds with 
long silky hairs, Gossypinm, Anemone, Asclepias, etc., and those with 
straight capillary hairs or bristles not confined to one end, Typha, Salix, etc. 

4. Parachute, petasospores. The highly developed members of this group, 
Taraxacum, Lactuca, and other Ligulitlorae are connected through Senecio 
and Eriophorum with the preceding. These represent the highest develop- 
ment of mobility attained by special modification. 

5. Chaffy-pappose, carphospores. In this group are placed those achenes 
with a more or less scaly or chaffy pappus with slight mobility, as in Rud- 
beckia, Brauneria, Helianthus, etc. 

6. Plumed, lophospores. In the fruits of this class, the style is the part 
usually modified into a long plumose organ, possessing a high degree of mo- 
bility, as in Pulsatilla, Sieversia, and Clematis. 



214 THE FORMATION 

7. Awned, acospores. These are almost exclusively grasses, in which the 
awns serve for distribution by wind, water, or animals, and even, according 
to Kerner. by hygroscopic creeping movements. The mobility in many cases 
is great. 

8. Spiny, centrospores. This group contains a few representatives which 
possess a moderate degree of mobility by attachment, as in Tribulus and 
Cenchrus. 

9. Hocked, oncospores. The members of this group are extremely numer- 
ous, and the degree of mobility as a rule is very high. All exhibit in com- 
mon the development of hooks or barbs, by which they are disseminated in 
consequence of attachment, though the number, size, and disposition of the 
hooks vary exceedingly. 

10. Viscid, gloeospores. In these, the inflorescence is more or less covered 
with a viscid substance, as in species of Silene, or the fruit is beset with 
glandular hairs, as in Ccrastium, Salvia, etc. 

11. Fleshy, sarcospores. These are intended for dissemination by deglu- 
tition, largely by birds ; the effectiveness of the modification depends in a 
large degree upon the resistance of the seed envelope to digestion. The 
mobility varies greatly, but the area over which migration may be effected is 
lar<^e. 

12. Nut-fruited, creatospores. This group includes those plants with nut 
fruits which are carried away and secreted by animals for food. 

13. Flagellate, mastigospores. These are plants with ciliate or flagellate 
propagative cells, i. e., zoogoniclia, as in Protococcus, Ulothrix, Oedogonium, 
Ectocarpus, etc., or with plant bodies similarly motile, Bacteriaceae and 
Volvocaceae. 

265. Position of disseminule. The position on the plant of the organ to 
be disseminated, i. e., its exposure to the distributing agent, plays a consid- 
erable part in determining the degree of mobility. In the majority of plants, 
the position of the inflorescence itself results in maximum exposure, but in a 
large number of forms special modifications have been developed for placing 
the spores or seeds in a more favorable position. In both cases, there are 
often present also devices for bringing about the abscission of the seed or 
fruit. It is, moreover, self-evident that the height of the inflorescence above 
ground or above the surrounding vegetation is likewise of considerable im- 
portance in increasing the trajectory. It is yet too early to make a complete 
classification of contrivances for placing disseminules in the most favorable 
exposure, but the following will serve as a basis for future arrangements. 

1. In all operculate Discomycetes, and especially in the Ascobolaceae, 
where the asci project above the hymenium, the spores are raised above the 



INVASION 215 

surface by tensions within the apothecium. This might be regarded as dis- 
semination by expulsion, if it were not for the fact that the spores fall back 
into the cup, unless carried away by the wind. 

2. In Gasteromycetes and in certain Hepaticae, the spores are not only 
elevated slightly above the sporophore by the expanding capillitium or by 
the mass of elaters, but they are also held apart in such a way that the wind 
blows them out much more readily. 

3. In Bryophyta, the sporophore regularly dehisces by a slit, or is pro- 
vided with a peristome. Both structures are for the purpose of sifting the 
spores out into the wind : by reason of their hygroscopicity, they also insure 
that the spores will not be shaken out in wet weather. 

4. In a few grasses, such as Stipa and Aristida, the twisting and inter- 
twining of the awns lift the floret out of the glumes, and at the same time 
constitute a contrivance readily blown away by the wind or carried by 
attachment. 

5. In certain Compositae, the involucral scales are reflexed at maturity, 
and at the same time the disk becomes more or less convex, serving to loosen 
the achenes. This result is also secured in certain species by the drying and 
spreading of the pappus hairs. 

6. The scapose LiguliHorae, Taraxacum, Agoscris, etc., are characterized 
by the elongation of the scape after anthesis, with the result that the head is 
raised to a considerable height by the time the achenes are mature. 

7. Carpotropic movements, though primarily for another purpose, often 
serve to bring seeds and fruits into a better position for dissemination. 

266. Seed production The relation of spore or seed-production to mobil- 
ity is obvious in the case of mobile species ; in the case of immobile ones, it 
is just as evident that it has no effect, though it may still have considerable 
influence in increasing migration. In the case of two species with equally 
effective dissemination contrivances, the one with the largest seed-production 
will be the more mobile. On the basis of the relation of seeds to flower, two 
groups of plants may be distinguished, one, Polyantliae, in which the flowers 
are many and the seeds few or single, as in Compositae, and the other, Poly- 
spermatac. Portulaca, Yucca, etc., in which the number of seeds to each 
flower is large. So far as the actual number of seeds produced is concerned, 
polyanthous plants may not differ from polyspermatous ones, but, as a rule, 
they are much more highly specialized for dissemination and are more mo- 
bile. The number of fertile seeds is also much greater, a fact which is of 
great importance in ecesis, and which, taken in connection with mobility, 
partially explains the supremacy of the composites. Among the fungi and 
algae, the amount of spore-production in a large degree determines the mo- 
bility, since these forms are intrinsically permobile. 



21 6 THE FORMATION 

267. Agents of migration. In the last analysis, however, the possibility ot 
migration depends upon the action of distributive agents ; in the absence of 
these, even the most perfect contrivance is valueless, while their presence 
brings about the distribution of the most immobile form. In short, migra- 
tion depends much more upon such agents than upon mobility, however per- 
fect the latter may be. It is, moreover, evident that the amount and extent 
of migration will be determined primarily by the permanence and forceful- 
ness of the agent, as indicated by its ability to bring about transportation. 
Finally, as will be shown later, the direction and rapidity of migration de- 
pend directly upon the direction and intensity of the agent. 

Migration results when spores, seeds, fruits, offshoots, or plants are moved 
out of their home by water, wind, animals, man, gravity, glaciers, growth, or 
mechanical propulsion. Corresponding to these agents, there may be recog- 
nized the following groups : 

1. Water, hydrochores. These comprise all plants distributed exclusively 
by water, whether the latter acts as ocean currents, tides, streams, or surface 
run-off. In the case of streams and run-off, especially, mobility plays little 
part, provided the disseminules are impervious or little subject to injury by 
water. Motile plants, or those with motile cells, which belong entirely to 
this group, may be distinguished as autochores, which correspond closely to 
mastigospores. 

2. Wind, anemochores. This group includes the majority of all permo- 
bile terrestrial plants, i. e., those in which modifications for increasing sur- 
face have been carried to the extreme, or those which are already permobile 
by reason of the minuteness of the spore or seed. Saccate, winged, comate, 
parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits 
represent the various types of modifications for wind-distribution. 

3. Animals, zoochores. Among terrestrial plants, dissemination by at- 
tachment represents essentially the same degree of specialization as is found 
in wind-distributed plants. The three types of contrivances for this purpose 
are found in spinose, hooked, and glandular fruits. Dissemination by 
deglutition and by carriage, either intentional or unintentional, though of 
less value, play a striking part on account of the great distance to which 
the seeds may be carried. Dissemination by deglutition is characteristic 
of sarcospores, and distribution by carriage of creatospores. 

4. Man, brotochores. Dissemination by man has practically no connec- 
tion with mobility. It operates through great distances and over immense 
areas as well as near at hand. It may be intentional, as in the case of 
cultivated species, or unintentional, as in thousands of native or exotic 
species. No other disseminating agent is comparable with man in respect 
to universal and obvious migration. 



INVASION 217 

5. Gravity, clitochores. The members of this group are exclusively col- 
line, montane, and alpine plants, growing on rocks, cliffs, and gravel-slides 
(talus), etc., in which the seeds reach lower positions merely by falling, or 
more frequently by the breaking away and rolling down of rock or soil 
masses and particles. Dissemination by this method is relatively insignifi- 
cant, though it plays an important part in the rock fields and gravel slides 
of mountain regions, particularly in the case of immobile species. 

6. Glaciers, crystallochores. At the present time, transport by glaciers 
is of slight importance, because of the restriction of the latter to alpine and 
polar regions, where the flora is poorly developed. In the consideration 
of migrations during the glacial epoch, however, it plays an important point. 

7. Growth, blastochores. The mobility of species disseminated by off- 
shoots is extremely slight, and the annual movement relatively insignificant. 
The certainty of migration and of. ecesis, is, however, so great, and the 
presence of offshoots so generally the. rule in terrestrial plants that growth 
plays an important part in migration, especially within formations. 

8. Propulsion, bolochores. Like growth, dissemination by mechanical 
propulsion, though operating through insignificant distances, exerts an im- 
portant effect in consequence of its cumulative action. The number of 
plants, however, with contrivances for propulsion is very much smaller than 
the number of blastochores. All bolochorous species agree in having modifi- 
cations by means of which a tension is established. At maturity, this 
tension suddenly overcomes the resistance of sporangium or fruit, and 
throws the enclosed spores or seeds to some distance from the parent plant. 
In accordance with the manner in which the tension is produced, sling-fruits 
may be classified as follows : 

(a) Hygroscopicity, pladoboles. These include the ferns with annulate 
sporangia, in which the expansion of the annulus by the absorption of mois- 
ture bursts the sporangium more or less suddenly, though the actual pro- 
pulsion of the spores seems to come later as a result of dessication. 

(b) Turgescence, edoboles. Dissemination by turgescence is highly 
developed in Pilobolus and in Discomycetes, though in the latter turgescence 
results rather in placing the spores in a position to be readily carried by 
the wind. Impatiens and Oxalis furnish familiar examples of fruits which 
dehisce in consequence of increased turgidity. 

(c) Dessication, xerioboles. The number of fruits which dehisce upon 
drying is very large, but only a small portion of these expel their seeds 
forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different 
ways in which dessication effects the sudden splitting of fruits. 

(d) Resilience, tonoboles. In some plants, especially composites, labiates, 
and borages, the achenes or nutlets are so placed in the persistent calyx or 



2l8 THE FORMATION 

involucre that the latter serves as a sort of mortar for projection, when the 
stem of the plant is bent to one side by any force, such as the wind or an 
animal. It will be noticed that two separate agents are actually concerned in 
dissemination of this sort. 

Frequently, two or more agents will act upon the same disseminule, 
usually in succession. The possibility of such combinations in nature is 
large, but actual cases seem to be infrequent, except where the activities of 
man enter into the question. Some parts, moreover, such as awned inflor- 
escences, are carried almost equally well by wind or animals, and may often 
be disseminated by the cooperation of these two agents. The wind also 
often blows seeds and fruits into streams by which they are carried away, 
but here again, parts adapted to wind-dissemination are injured as a rule 
by immersion in water, and the number of plants capable of being scattered 
by the successive action of wind and water is small. 

In the present state of our knowledge of migration, it is impossible to 
establish any definite correspondence between dissemination-contrivance, 
agent, and habitat. As a general rule, plants growing in or near the water, 
in so far as they are modified for this purpose at all, are adapted to water- 
carriage. Species which grow in exposed grassy or barren habitats are for 
the most part anemochores, while those that are found in the shelter of 
forests and thickets are usually zoochorous, though the taller trees and 
shrubs, being exposed to the upper air currents, are generally wind-distrib- 
uted. There is then a fair degree of correspondence, inasmuch as most 
hydrophytes are hydrochorous, most hylophytes, zoochorous, and the ma- 
jority of poophytes and xerophytes, anemochorous. Definite conclusions 
can be reached, however, only by the statistical study of representative 
formations. 

With respect to their activity, agents may be distinguished as constant, 
as in the case of currents, strea'ms, winds, slope, growth, and propulsion, 
or intermittent, animals and man. In the former, the direction is more or 
less determinate, and migration takes place year by year, i. e., it is contin- 
uous, while in the latter dissemination is largely an accidental affair, inde- 
terminate in direction, and recurring only at indefinite intervals. The 
effective conversion of migration into invasion is greatest when the move- 
ment is continuous, and least when it is discontinuous, since, in the latter, 
species are usually carried not only out of their particular habitat but even 
far beyond their geographical area, and the migration, instead of being 
an annual one with the possibility of gradual adjustment, may not recur for 
several years, or may, indeed, never take place again. The rapidity of 
migration is greatest in the case of intermittent agents, while the distance 
of migration is variable, being great chiefly in the case of man, ocean-cur- 



INVASION 219 

rents, and wind, and slight when the movement is due to slope, growth, or 
propulsion. Disregarding the great distances over which artificial trans- 
port may operate, seeds may be carried half way across the continent in a 
week by strong-flying birds, while the possibilities of migration by growth 
or expulsion are limited to a few inches, or at most to a few feet per year. 
This slowness, however, is more than counterbalanced by the enormously 
greater number of disseminules, and their much greater chance of becoming 
established. • 

268. The direction of migration is determinate, except in the case 
of those distributive agents which act constantly in the same direction. 
The general tendency is, of course, forward, the lines of movement radiating 
in all directions from the parent area. This is well illustrated by the opera- 
tion of winds which blow from any quarter. In the case of the constant 
winds, migration takes a more or less definite direction, the latter being 
determined to a large degree by the fruiting period of any particular species. 
In this connection, it must be kept clearly in mind that the position of new 
areas with reference to the original home of a species does not necessarily 
indicate the direction of migration, as the disseminules may have been 
carried to numerous other places in which ecesis was impossible. The local 
distribution of zoochorous species is of necessity indeterminate, though 
distant migration follows the pathways of migratory birds and animals. In 
so far as dissemination by man takes place along great commercial routes, 
or along highways, it is determinate. In ponds, lakes, and other bodies of 
standing water, migration may occur in all directions, but in ocean currents, 
streams, etc., the movement is determinate, except in the case of motile 
species. The dissemination of plants by slopes, glaciers, etc., is local and 
definite, while propulsion is in the highest degree indeterminate. Migration 
by growth is equally indefinite, with the exception that hydrotropism and 
chemotropism result in a radiate movement away from the mass, while 
propulsion throws seeds indifferently into or away from the species-mass. 
From the above it will be seen that distant migration may take place by 
means of water, wind, animals or man, and, since all these agents act in 
a more or less definite direction over great distances, that it will be in 
some, degree determinate. On the other hand, local migration will as 
regularly be indeterminate, except in the case of streams and slopes. The 
direction of migration, then, is controlled by these distributive agents, and 
the limit of migration is determined by the intensity and duration of the 
agent, as well as by the character of the space through which the latter 
operates. 



220 THE FORMATION 

ECES/S 

269. Concept. By the term ecesis is designated the series of phenomena 
exhibited by an invading disseminule from the time it enters a new forma- 
tion until it becomes thoroughly established there. In a word, ecesis is 
the adjustment of a plant to a new habitat. It comprises the whole process 
covered more or less incompletely by acclimatization, naturalization, accom- 
modation, etc. It is the decisive factor in invasion, inasmuch as migra- 
tion is entirely ineffective without it, and is of great value in indicating the 
presence and direction of migration in a great number of species where 
the disseminule is too minute to be detected or too little specialized to be 
recognizable. 

The relation of migration to ecesis is a most intimate one : the latter 
depends in a large measure upon the time, direction, rapidity, distance, and 
amount of migration. In addition, there is an essential alternation between 
the two, inasmuch as migration is followed by ecesis, and the latter then 
establishes a new center from which further migration is possible, and so on. 
The time of year in which fruits mature and distributive agents act has a 
marked influence upon the establishment of a species. Disseminules designed 
to pass through a resting period are often brought into conditions where 
they germinate at once, and in which they perish because of unfavorable 
physical factors, or because competing species are too far advanced. On 
the other hand, spores and propagules designed for immediate germination 
may be scattered abroad at a time when conditions make growth impossible. 
The direction of movement is decisive in that the seed or spore is carried 
into a habitat sufficiently like that of the parent to secure establishment, 
or into one so dissimilar that germination is impossible, or at least is not 
followed by growth and reproduction. The rapidity and distance of migra- 
tion have little influence, except upon the less resistant disseminules, conidia, 
gemmae, etc. Finally, the amount of migration, i. e., the number of mi- 
grants, is of the very greatest importance, affecting directly the chances 
that vigorous disseminules will be carried into places w r here ecesis is possible. 

Normally, ecesis consists of three essential processes, germination, growth, 
and reproduction. This is the rule among terrestrial plants, in which mi- 
gration regularly takes place by means of a resting part. In free aquatic 
forms, however, the growing plant or part is usually disseminated, and 
ecesis consists merely in being able to continue growth and to insure re- 
production. Here establishment is practically certain, on account of the 
slight differences in aquatic habitats, excepting of course the extremes, fresh 
water and salt water. The ease indeed with which migration and ecesis are 
effected in the water often makes it impossible to speak properly of invasion 
in this connection, since aquatics are to such a large extent cosmopolitan. 



INVASION 221 

In dissemination by offshoots, the conditions are somewhat similar. Here, 
also, ecesis comprises the sequence of growth and reproduction, and in- 
vasion, in the sense of passing- from one habitat to another, is of rare 
occurrence, as the offshoot grows regularly under the same conditions as 
the parent plant. The adjustment of growing plants and parts is so slight, 
and their establishment so certain on account of their inability to migrate 
into very remote or different habitats, that they may be ignored in the fol- 
lowing discussion. 

In accordance with the above, it would be possible to distinguish three 
groups of terrestrial plants: (i) those migrants which germinate and dis- 
appear, (2) those which germinate and grow but never reproduce, (3) 
those which reproduce, either by propagation or generation, or both. Such 
a classification has little value, however, since the same species may behave 
in all three fashions, depending upon the habitat to which it has migrated, 
and since invasion does not occur unless the plant actually takes possession, 
i. e., reproduces. From the latter statement, it follows that invasion occurs 
only when a species migrates to a new place, in which it germinates, ma- 
tures, and reproduces. Maintenance by annual invasion simply, in which 
the plants of each year disappear completely, can not then be regarded 
as invasion proper. On the other hand, though such instances are rare, it 
is not necessary that the invaders produce fruit, provided they are able to 
maintain themselves, or to increase by propagation. Furthermore, if a plant 
germinate, grow, and reproduce, it is relatively immaterial whether it per- 
sist for a few years or for many, since, as we shall see under Succession, 
the plants of one invasion are displaced by those of the next, the interval 
between invasions increasing with the stabilization. 

270. Germination of the seed. The germination of seed or spore 
is determined by its viability and by the nature of the habitat. Viability 
depends upon the structural characters of fruit, seed-coat, and endosperm, 
and to a degree upon the nature of the protoplasm or embryo. The first 
three affect the last directly, by protecting the embryo against dryness, 
against injury due to carriage by water, or by deglutition, and probably in 
some cases against excessive heat' or cold. Marloth 1 has investigated the 
structure of seed coats, establishing the following groups, which are sum- 
marized here somewhat fully because of their bearing upon ecesis: (1) 
seed coats without protective elements, endosperm absent or rudimentary, 
Epilobiunij Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements 
lacking or few, endosperm highly developed with thick-walled cells, 

l Uber mechanische Shutzmittel der Samen gegen schadliche Einfliisse von ausscn. 
Engler Bot. Jahrb., 5 :56. 1883. 



222 THE FORMATION 

Liliaceae, Primulaceae, Rubiaceae, etc.; (3) protective cells present in the 
seed coats, endosperm little or none, Boraginaceae, Crassulaceae, Cruciferae, 
Labiatae, Papilionaceae, etc.; (4) protective elements present, Asclepias, 
Campanula, Gentiana, Silcne, Saxifraga, etc.; (5) protective cells present, 
endosperm thick-walled, Euonymus, Helianthemum, Ribes. The protective 
cells are of various kinds : ( 1 ) epidermal cells strongly cuticularized, 
Caryophyllaceae, Crassulaceae, Fumariaceae, Saxifragaceae; (2) paren- 
chyma thick-walled, several-layered, Aesculus, Castanea, Fagus; (3) 
.parenchyma cells with the inner or radial walls thickened, Campanula, 
Erythraea, Gentiana; (4) epidermal cells cup-shaped, thick-walled, Cruci- 
ferae, Ribes, Vaccinium; (5) parenchyma with thickened, cellulose walls, 
Geranium, Viburnum; (6) a single row of stone-cells, Labiatae; (7) tissue 
of stone- cells, Hippuris, Naias, Potamogeton; (8) elongate stone-cells, 
Coniferae, Cupuliferae, Euphorbia, Linum, Malva, Viola; (9) short, colum- 
nar, thick-walled branched cells, Cucurbitaccae, Datura, Hypericum; (10) 
prosenchyma with cellulose walls, Clematis; (11) prosenchyma with lignified 
walls, Fraxinus, Rhamnus, Ranunculus. The seed-coats have a certain in- 
fluence in determining germination at the proper time, inasmuch as they 
make it difficult for the seed to germinate under the stimulus of a quantity 
of warmth and moisture insufficient to support the seedling. The effect of 
the endosperm, as well as that of other food-supply in the seed, upon 
germination and the establishment of the seedling is obvious. 

The behavior of seed or spore with respect to germination depends in 
a large degree upon the character of the protoplasm or embryo, though in 
just what way is at present a matter of conjecture. It is evident that many 
seeds are not viable because fertilization has not been effected, and in con- 
sequence no embryo has developed. This is the usual explanation of the 
low germinating power of the seeds of some species, especially polysperma- 
tous ones. But even in viable seeds the behavior is always more or less 
irregular. The seeds of some species will grow immediately after ripening, 
while others germinate only after a resting period of uncertain duration. 
The same is true of spores. Even in the case of seeds from the same 
parent, under apparently similar conditions, while the majority will germi- 
nate the first year, some will lie dormant for one or more years. The 
precise reason why m.any seeds and spores germinate more readily after 
being frozen is equally obscure. The period of time for which disseminules 
may remain viable is extremely diverse, though, as would be expected, it 
is much longer as a rule for seeds than for spores. The greater vitality of 
seeds in the case of ruderal plants suggests that this diversity may be due 
simply to variation in the vigor of the embryos. It would seem that under 
proper conditions seeds may retain their viability for an indefinite period. 



INVASION 223 

The influence of habitat upon germination is of primary importance, 
though the manner in which its influence is exerted is by no means as 
evident as might be supposed. In the case of seeds sown in the planthouse, 
it is almost universally the case that germination is less than in nature, 
notwithstanding the fact that temperature and moisture appear to be 
optimum. In nature, the seeds of the species may be carried into a number 
of different formations, any one or all of which may present conditions un- 
favorable to germination. With respect to probability of germination, 
habitats are of two sorts : those which are denuded and those which bear 
vegetation. It is impossible to lay down general propositions with respect 
to either group, since germination will vary with the character of the in- 
vading species, the annual distribution of heat and moisture in the habitat, 
etc. In a general way, however, it may be stated that the chances for 
germination are greater in vegetation than in denuded areas, chiefly because 
the latter are usually xerophytic. On the other hand, the lack of competi- 
tion in the denuded area tends to make ultimate establishment much more 
certain. Here, as elsewhere when exact statistical results are desired, the 
use of the quadrat, and especially of the permanent quadrat, is necessary 
to determine the comparative germination of the invading species in relation 
to denudation and vegetation. 

271. Adjustment to the habitat. The seedling once established by 
germination, the probability of its growing and maturing will depend upon 
its habitat form, plasticity, and vegetation form. Even though it may 
germinate under opposite conditions, a typical hylophyte, such as Impatiens 
for example, will not thrive in an open meadow, nor will characteristic 
poophytes, such as most grasses, grow in deep shade. In the same way, 
xerophytes do not adapt themselves to hydrophytic habitats, nor hydrophytes 
to xerophytic conditions. Many mesophytes, however, possess to a certain 
degree the ability to adjust themselves to somewhat xerophytic or hydro- 
phytic situations, while woodland plants often invade either forest or 
meadow. This capability for adjustment, i. e., plasticity, is greatest in in- 
termediate species, those that grow in habitats not characterized by great 
excess or deficiency of some factor, and it is least in forms highly specialized 
in respect to water-content, shade, etc. It may then be established as a 
fundamental rule that ecesis is determined very largely by the essential physi- 
cal similarity of the old and the new habitat, except in the case of plastic 
forms, which admit of a wider range of accomodation. The plasticity of 
a plant is not necessarily indicated by structural modification, though such 
adjustment is usually typical of plastic species, but it may sometimes arise 
from a functional adaptation, which for some reason does not produce 






224 THE FORMATION 

concomitant structural changes. The former explains such various habitat 
forms of the same species as are found in Galium bore ale, Gentiana acuta, 
etc., and the latter the morphological constancy of plants like Chamaenerium, 
which grow in very diverse habitats. 

The vegetation form of the invading species is often of the greatest im- 
portance in determining- whether it will become established. The vegetation 
form represents those modifications which, produced in the original home 
by competition, i. e., the struggle for existence, are primarily of value in 
securing and maintaining a foothold. These comprise all structures by 
means of which the plant occupies a definite space in the air, through which 
the necessary light and heat reach it, and in the soil, from which it draws 
its food supply. These structures are all organs of duration or of peremp- 
tion, such as root, rootstalk, bulb, tuber, woody stem, etc., which find their 
greatest development among trees and shrubs, and their least among annual 
herbs. But while the invaders are aided in securing possession' by the proper 
vegetation form, the occupation of the plant already in possession is in- 
creased by the same means, and the outcome is then largely determined by 
other factors. To avoid repetition, the bearing of occupation upon invasion 
will be considered under succession. 

BARRIERS 

272. Concept. DeCandolle 1 seems to have been the first to use the term 
barrier and to distinguish the various kinds, though Hedenberg 2 clearly saw 
that stations of one kind were insurmountable obstacles to plants belonging 
to a very different type. De Candolle pointed out that the natural barriers 
to continuous invasion ("transport de proche en proche") are: (i) seas, 
which decrease invasion almost in inverse proportion to their extent; (2) 
deserts; (3) mountain ranges, which are less absolute on account of passes, 
valleys, etc.; (4) vegetation, marshes being barriers to dry land plants, 
forests to those that fear the shade, etc. Grisebach 8 , in discussing the effect 
of barriers upon the constitution of vegetation, laid down the fundamental 
rule that: "The. supreme law which serves as the basis of the permanent 
establishment of natural floras is to be recognized in the barriers which have 
hindered or completely prevented invasion." 

Any feature of the topography, whether physical or biological, that re- 
stricts or prevents invasion, is a barrier. Such features are usually perma- 
nent and produce permanent barriers, though the latter may often be 
temporary, existing for a few years only, or even for a single season. In 

x Essai Elementaire de Geographie Botanique, 45. 1820. 
2 Stationes Plantarum Amoen. Acad., 4:64. 1754. 
3 Die Vegetation der Erde, 4. 1872. 



INVASION 225 

this last case, however, they are as a rule recurrent. Barriers may further- 
more be distinguished as complete or incomplete with respect to the 
thoroughness with which they limit invasion. Finally, the consideration of 
this subject gains clearness if it be recognized that there are barriers to 
migration as well as to ecesis, and if we distinguish barriers as physical or 
biological with reference to the character of the feature concerned. 

273. Physical barriers are those in which limitation is produced by 
some marked physiographic feature, such as the ocean or some other large 
body of water, large rivers, mountain ranges and deserts (including ice 
and snow fields). All of these are effective by virtue of their dominant 
physical factors ; hence they are barriers to the ecesis of species coming from 
Very different habitats, but they act as conductors for species from similar 
vegetation, especially in the case of water currents. A body of water, repre- 
senting maximum water-content, is a barrier to mesophytic and xerophytic 
species, but a conductor for hydrophytic ones ; deserts set a limit to the 
spread of mesophytic and hydrophytic plants, while they offer conditions 
favorable to the invasion of xerophytes ; and a high mountain range, be- 
cause of the reduction of temperature, restricts the extension of macrother- 
mal and mesothermal plants. A mountain range, unlike other physical 
barriers, is also an obstacle to migration, inasmuch as natural distributive 
agents rarely act through it or over it. 

274. Biological barriers include vegetation, man and animals, and plant 
parasites. The limiting effect of vegetation is exhibited in two ways. 
In the first place, a formation acts as a barrier to the ecesis of species in- 
vading it from the formations of another type, on account of the physical 
differences of the. habitats. Whether such a barrier be complete or partial 
will depend upon the degree of dissimilarity existing between the forma- 
tions. Hylophytes are unable to invade a prairie, though open thicket plants 
may do so to a certain degree. In the same way, a forest formation on 
account of its diffuse light is a barrier to poophytes ; and a swamp, because 
of the amount and character of the water-content, sets a limit to both 
hylophytes and poophytes. Formations, such as forests, thickets, etc., 
sometimes act also as direct obstacles to migration, as in the case of tumble- 
weeds and other anemochores, clitochores, etc. A marked effect of vege- 
tation in decreasing invasion arises from the closed association typical of 
stable formations and of social exclusive species. In these, the occupation 
is so thorough and the struggle for existence so intense that the invaders, 
though fitted to grow under the physical factors present, are unable to com- 
pete with the species in possession for the requisite amount of some neces- 



226 THE FORMATION 

sary factor. Closed associations usually act as complete barriers, while open 
ones restrict invasion in direct proportion to the degree of occupation. To 
this fact may be traced a fundamental law of succession, viz., the number 
of stages in a succession is determined largely by the increasing difficulty 
of invasion as the habitat becomes stabilized. Man and animals affect mi- 
gration directly, though not obviously, by the destruction of disseminules. 
They operate as a pronounced barrier to ecesis wherever they alter condi- 
tions in such a way as to make them unfavorable to invading species, or 
when, by direct action upon the latter, such as grazing, tramping, parasit- 
ism, etc., they turn the scale in the struggle for existence. The absence of 
insects adapted to insure fertilization is sometimes a serious barrier to the 
establishment of adventitious or introduced plants. The presence of para- 
sitic fungi, in so far as they destroy the seeds of plants, acts as an obstacle 
to migration, and restricts or prevents ecesis in so far as the fungi destroy 
the invaders, or place them at a disadvantage in the struggle for existence. 

275. Influence of barriers. Physical barriers are typically permanent in 
character, while biological ones are either permanent or temporary, depend- 
ing upon the permanence of the formation and the constancy of the physical 
factors which determine it. A stable formation, such as a forest or meadow, 
which acts as a decided barrier to invasion from adjacent vegetation, may 
disappear completely, as a result of a landslide, flood, or burn, or through 
the activity of man, and may leave an area into which invaders crowd from 
every point. Often, without undergoing marked change, a formation which 
has presented conditions unfavorable to the ecesis of species of mesophytic 
character may, by reason of a temporary change in climate, become suffi- 
ciently modified to permit the invasion of mesophytes. On the other hand, 
a meadow ceases to be a barrier to prairie xerophytes during a period of 
unusually dry years. A peculiar example of the modification of a barrier is 
afforded by the defoliation of aspen forests in the mountains as a result of 
which poophytes have been enabled to invade them. Nearly all xerophytic 
stretches of sand and gravel, dunes, blowouts, gravel slides, etc., and even 
prairies to a certain degree, exhibit a recurrent seasonal change in spring, 
as a result of which the hot, dry surface becomes sufficiently moist to per- 
mit the germination and growth of invaders, which are entirely barred out 
during the remainder of the year. In an absolute sense, no barrier is com- 
plete, since the coldest as well as the dryest portions of the earth's surface 
are capable, at times at least., of supporting the lowest types of vegetation. 
Relatively, however, in connection with the natural spread of terrestrial 
plants, it is possible to distinguish partial barriers from complete ones. 
Such a distinction is of importance in the consideration of invasions from 



INVASION 227 

a definite region, as it is only in this restricted sense that complete barriers 
have produced endemism. 

Distance, though hardly to be considered a barrier in the strict sense of 
the word, unquestionably plays an important part in determining the amount 
of invasion. The effect of distance is best seen in the case of migration, as 
it influences ecesis only in those rare cases where viability is affected. The 
importance of distance, or take the converse, of proximity, is readily ascer- 
tained by the study of any succession from denudation. It has been es- 
tablished that the contiguous vegetation furnishes 75-90 per cent of the 
constituent species of the initial formation, and in mountainous regions, 
where ruderal plants are extremely rare, the percentage is even higher. The 
reason for this is to be found not only in the fact that the adjacent species 
have a much shorter distance to go, and hence will be carried in much 
greater quantity, but also in that the species of the formations beyond must 
pass through or over the adjacent ones. In the latter case, the number of 
disseminules is relatively small on account of the distance, while invasion 
through the intermediate vegetation, if not entirely impossible, is extremely 
slow, so that plants coming in by this route reach the denuded area only 
to find it already occupied. It is as yet impossible to give a definite numeri- 
cal value to proximity in the various invasions that mark any particular 
succession. This will not be feasible until a satisfactory method has been 
found for determining a coefficient of mobility, but, this once done, it will 
be a relatively simple matter, not merely to trace the exact evolution of any 
succession of formations, but actually to ascertain from the adjacent vege- 
tation the probable constitution of a particular future stage. 

From what has been said, it follows that the primary effect of barriers 
upon vegetation is obstruction. Where the barrier is in the pathway of 
migration, however, it causes deflection of the migrant as a rule, and sets 
up migration in a new direction. This is often the case when the strong 
winds of the plains carry disseminules towards the mountains and, being 
unable to cross the range, drop them at the base, or, being deflected, carry 
them away at right angles to the original direction. The same thing happens 
when resistant fruits and seeds borne by the wind fall into streams of water 
or into ocean currents. The direction of migration is changed, and what is 
normally a barrier serves as an agent of dissemination. 

ENDEMISM 

276. Concept. Since its first use by DeCandolle, the term endemic has 
been employed quite consistently by phytogeographers with the meaning 
of "peculiar to a certain region." Some confusion, however, has arisen 
from the fact that a few authors have made it more or less synonymous 



228 THE FORMATION 

with indigenous and autochthonous, while others have regarded it as an 
antonym of exotic. In its proper sense, endemic refers to distribution, and 
not to origin. Its exact opposite will be found then in Fenzl's term poly- 
demic, dwelling in several regions. Indigenous (autochthonous) and exotic, 
on the contrary, denote origin, and are antonyms, indigenous signifying 
native, and exotic foreign. As Drude has shown, endemic plants may be 
either indigenous, as in the case of those species that have never moved out 
of the original habitat, or exotic, as in the much rarer instances where a 
polydemic species has disappeared from its original home and from all 
regions into which it has migrated except one. It is understood that not 
all indigenous or exotic species are endemic. The proportion of endemic to 
polydemic species is a variable and somewhat artificial one, depending upon 
the size of the divisions employed. 

277. Causes. The primary causes of endemism are two, lack of migra- 
tion and presence of barriers. Since distributive agents are practically 
universal, lack of migration corresponds essentially to immobility, a fact 
which decreases the difficulty of ascertaining the immediate causes of 
endemism in any particular species. Either immobility or a barrier may 
produce endemism: extremely immobile plants, for example, liliaceous 
species propagating almost wholly by underground parts, are as a rule 
endemic, while alpine plants and those of oceanic islands are endemic in 
the highest degree, regardless of their mobility. When the two conditions 
act concomitantly upon a species, endemism is almost inevitable. It can not 
be supposed, however, that immobility or natural barriers alone, or the 
concomitance of the two, must invariably give rise to endemic species ; the 
most immobile plant may be carried into another region by unusual or 
accidental agencies, or the most formidable barrier to migration may be 
overcome by the intensity of an agent or through the action of man. En- 
demism is also brought about by the modification of species ; new or 
nascent species are as a rule endemic. Whether they will remain endemic 
or not will depend upon the perfection of their contrivances for dissemina- 
tion and upon the presence of barriers to migration or ecesis. Finally, as 
Drude was the first to point out, the disappearance of a polydemic species in 
all regions but one, owing to the struggle for existence or to changed 
physical conditions, will result in endemism. 

278. Significance. Endemism is readily recognized by methods of 
distributional statistics, applied to areas limited by natural barriers to 
migration or ecesis. For political areas, it has no significance whatever, 
unless the boundaries of these coincide with barriers. It determines in the 



INVASION 229 

first degree the validity of regions, though the latter are often recognized 
also by the presence of barriers and by the character of the vegetation. 
Endemism may occur in areas of vegetation of any rank from a formation 
to a zone. When the term is not qualified, however, it should be used of 
species with reference to formations alone. Comparisons to be of value, 
however, can be instituted only between areas of the same order, i. e., be- 
tween two or more formations, two or more regions, provinces, etc. In the 
same way, taxonomic groups of the same rank should be used in such com- 
parisons, i. e., species should be contrasted with species, genera with genera, 
and families. with families, except when it is desired to obtain some measure 
of the age of the vegetation by the differentiation of the endemic phyla 
within it. There will be seen to exist a fundamental correspondence between 
the rank of the floral division and the taxonomic group, though the appar- 
ent exceptions to this are still too numerous to warrant its expression in a 
general law. As a rule, however, formations most frequently show endemic 
habitat forms and species, more rarely endemic genera ; regions and prov- 
inces commonly exhibit endemic species and genera, rarely endemic families ; 
while zones and hemispheres contain endemic orders as well as families. 
This correspondence is readily seen to depend primarily upon the fact that 
increased differentiation in the taxonomic sense is a concomitant of the in- 
creased invasion of endemic species, measured in terms of distance and dif- 
ference in habitat. 

It is too early to decide satisfactorily whether it is proper to speak of 
formations as endemic. At first thought it would seem that all formations, 
with the exception of ruderal ones, were endemic, but a study of almost 
any transition area between regions would seem to point to the opposite con- 
clusion, viz., that no formations are properly endemic. It is equally 
impossible at present to distinguish different types of endemics, such as 
relictae, etc., as any such classification must await the elaboration of a method 
for determining the phylogeny of a natural group of species by an investi- 
gation of their comparative differentiation in connection with their migration 
in all directions from the vegetation center into new habitats. In short, it 
will not be possible to make a thorough study of endemism and to postulate 
its laws until modern methods of research have been extended to a much 
larger portion of the vegetation of the globe. The final task of phytogeog- 
raphy is the division of the earth's vegetation into natural areas. It will 
be at once evident that most plants can not properly be called endemic until 
the natural regions in which they are found have been accurately defined, 
a work which has barely begun. In the much simpler matter of distribution, 
upon wdiich the accuracy of statistical methods depends directly, there are 
few regions sufficiently well known at the present time to yield anything 
like permanent results. 



230 



THE FORMATION 



POLYPHYLES/S AND POLYGENESIS 

279. Concept. The idea of polyphylesis, as advanced by Engler, con- 
tains two distinct concepts : ( I ) that a species may arise in two different 
places or at two different times from the same species, and (2) that a genus 
or* higher group may arise at different places or times by the convergence 
of two or more lines of origin. It is here proposed to restrict polyphylesis, 
as its meaning would indicate, to the second concept, and to employ for 
the first the term polygenesis, 1 first suggested by Huxley in the sense of 
polyphylesis. The term polyphylesis is extended, however, to cover the 
origin of those species which arise at different places or times from the 
convergence of two or more different species, a logical extension of the 
idea underlying polyphyletic genera, though it may seem at first thought to 
be absurd. Polygenesis may be formally defined as the origin of one species 
from another species at two or more distinct places on the earth's surface, 
at the same time or at different times, or its origin in the same place at 
different times. Polyphylesis, on the contrary, is the origin of one species 
from two or more different species at different places, at the same time or at 
different times. It is evident that what is true of species in this connection 
will hold equally well of genera and higher groups. Opposed to polygenesis 
is monogenesis, in which a species arises but once from another species ; with 
polyphylesis is to be contrasted monophylesis, in which the species arises 
from a single other species. It will be noticed at once that these two concepts 
are closely related. The following diagrams will serve to make the above 
distinctions more evident : 



I. Polygenesis 



II. Polyphylesis 




a 



8 a 



7fl n 




a 



III. Monogenesis 
(Monophylesis) 



a 




1 When this word was first proposed, the author did not know that Briquet had al- 
ready applied the term polytopism to this concept (Ann. Conserv. Bot. Gen., 5:73. 



INVASION 23I 

In I, a species A, becomes scattered over a large area in a series of places, 
m . . . m n y with the same physical factors, in any or all of which may 
arise the new species a. In II, a species with xerophytic tendency, A, and 
one with mesophytic tendency, B, in the course of migration find themselves 
respectively in a more mesophytic habitat, m, and a more xerophytic one, x, 
in which either may give rise to the new form, c, which is more or less 
intermediate between A and B. In III, the method of origin is of the 
simplest type, in which a species is modified directly into another one, or is 
split up into several. 

280. Proofs of polygenesis. In affirming the probability of a polygenic 
origin of species, there is no intention of asserting that all species originate 
in this way. It seems evident that a very large number of species of re- 
stricted range are certainly monogenetic, at least as far as origin in space 
is concerned. It is possible that any species may arise at two or more distinct 
times. Polygenesis can occur readily only in species of more or less ex- 
tensive area, in which recur instances of the same or similar habitat. The 
relative frequence and importance of the two methods can hardly be con- 
jectured as yet, but origin by monogenesis would seem to be the rule. 

The arguments adduced by Engler in support of polygenesis are in 
themselves conclusive, but the investigations of the past decade have brought 
to light additional proofs, especially from the experimental side. In de- 
termining the physical factors of prairie and mountain formations, and 
especially by methods of experimental ecology, the author has found that 
habitats are much less complex than they are ordinarily thought to be, 
since water-content and humidity, and to a less degree light, constitute the 
only factors which produce direct modification. In addition, it has been 
ascertained that the minimum difference of water-content, humidity, or light, 
necessary to produce a distinguishable morphological adjustment is much 
greater than the unit differences recorded by the instruments. In short, 
the differences of habitats, as ascertained by thermograph, psychrometer 
and photometer, are much greater than their efficient differences, and, with 
respect to their ability to produce modification, habitats fall into relatively 
few categories. A striking illustration of this is seen in the superficially 
very different habitats, desert, strand, alkali plain, alpine moor, and arctic 
tundra, all of which are capable of producing the same type of xerophyte. 
It follows from this that many more or less plastic species of extensive 

1901). Since polygenesis expresses the idea of origin, and applies to multiple origin 
in time as well as in space, it is retained as the name of this concept. Polytopic and 
monotopic are adopted for multiple and single origin in space respectively, and poly- 
chronic and monochrome are proposed for similar origin in time. 



232 THE FORMATION 

geographical area will find -themselves in similar or identical situations, 
measured in terms of efficient differences, and will be modified in the 
same way in two or more of these. In mountain regions, where interrup- 
tion of the surface and consequent alternation are great, the mutual invasion 
of contiguous formations is of frequent occurrence, often resulting in 
habitat forms. The spots in which these nascent species, such as Galium 
boreale hylocolum, Aster Levis lochmocolus, etc., are found, are often so 
related to the area of the parent species as to demonstrate conclusively 
that these forms are the result of polygenesis and not of migration. Na- 
turally, what is true of a small area will hold equally well of a large region, 
and the recurrence of the same habitat form may be accepted as conclusive 
proof of polygenesis. The most convincing evidences of multiple origin, 
however, are to be found in what DeVries has called "mutations." It 
makes little difference whether we accept mutations in the exact sense of this 
author, or regard them as forms characterized by latent variability. The 
evidence is conclusive that the same form may arise in nature or in cultiva- 
tion, in Holland or in America, not merely once, but several or many times. 
In the presence of such confirmation, it is unnecessary to accumulate proofs. 
Polygenesis throws a new light upon many difficult problems of invasion 
and distribution, and, as a working principle, admits of repeated tests in 
the field. It obviates, moreover, the almost insuperable difficulties in the 
way of explaining the distribution of many polygenetic species on the basis 
of migration alone. 

281. Origin by polyphylesis. In 1898, the author first advanced a ten- 
tative hypothesis to the effect that a species homogeneous morphologically 
may arise from two distinct though related species. During subsequent 
years of formational study, the convinction has grown in regard to the prob- 
ability of such a method of origin. Since the appearance of Engler's work, 
a polyphyletic origin for certain genera has been very generally accepted 
by botanists, but all have ignored the fact that the polyphylesis of genera 
carries with it the admission of such origin for species, since the former 
are merely groups of the latter. I can not, however, agree with Engler, that 
polyphyletic genera, and hence species also, are necessarily unnatural. If 
the convergence of the lines of polyphylesis has been great, resulting in 
essential morphological harmony, the genus is a natural one, even though 
the ancestral phyla may be recognizable. If, on the other hand, the con- 
vergence is more or less imperfect, resulting in subgroups of species more 
nearly related within the groups than between them, the genus can hardly 
be termed natural. This condition may, however, prevail in a monophyletic 
genus with manifest divergence and still not be an indication that it is 
artificial. 



INVASION 233 

Darwin 1 , in speaking of convergence, has said: "If two species, belong- 
ing to two distinct though allied genera, had both produced a large number 
of new and divergent forms, it is conceivable that these might approach each 
other so closely that they would have all to be classified under the same 
genus ; and thus the descendants of two distinct genera would converge 
into one.'' The application of this statement to species would at once show 
the possibility of polyphylesis in the latter, and a further examination of 
the matter will demonstrate its probability. It is perfectly evident that a 
species may be split into two or more forms by varying the conditions, let 
us say of water-content, and that the descendants of these forms may again 
be changed into the parent type by reversing the process. This has, in 
fact, been done experimentally. Since it is admittedly impossible to draw 
any absolute line between forms, varieties, and species, it is at once clear that 
two distinct though related species, especially if they are plastic, may 
be caused to converge in such a way that the variants may constitute 
a new and homogeneous species. This may be illustrated by a concrete 
case at present under investigation. Kuhnistera purpurea differs from 
K. Candida in being smaller, in having fewer, smaller, and more narrow 
leaflets, and a globoid spike of purple flowers in place of an elongated 
one of white flowers ; in a word, it is more xerophytic. This conclusion 
is completely corroborated by its occurrence. On . dozens of slopes 
examined, Kuhnistera purpurea has never been found mingling with K. 
Candida on lower slopes, except where an accident of the surface has resulted 
in a local decrease of water-content. The experiment as conducted is a 
simple one, consisting merely in sowing seed of each in the zone of the 
other, and in growing K. purpurea under controlled mesophytic conditions^ 
and K. Candida under similarly measured xerophytic conditions in the plant- 
house. 

While the polyphyletic origin of species is in a fair way to be decided by 
experiment, it receives support from several well-known phenomena. The 
striking similarity in the plant body of families taxonomically so distinct as 
the Cactaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and Jun- 
caceae, indicates that a vegetation form may be polyphyletic. On the other 
hand, the 'local appearance of zygomorphy, of symphysis, and of aphanisis 
in the floral types of phylogenetically distinct families is a proof of the 
operation of convergence in reproductive characters. To be sure, the con- 
vergence is never so great as to produce more than superficial similarity, 
but this is because the groups are markedly different in so many fundamental 
characters. The same tendency in closely related species would easily result 

x The Origin of Species, 1SG. 1859. 



234 THE FORMATION 

in indentity. As in the case of polygenesis, the relatively small number of 
typically distinct habitats makes it clear that two different species of wide 
distribution, bearing to each other the relations of xerophyte to mesophyte, 
of hydrophyte to mesophyte, or of poophyte to hylophyte, might often find 
themselves in reciprocal situations, with the result that they would give rise 
to the same new form. The final proof of the polyphylesis of species is 
afforded by the experiments of DeVries in mutation. DeVries found that 
Oenothera nanella arose from 0. Lamarckiana, O. laevifolia, and 0. scintil- 
lans; Oenothera scintillans arose from O. lata and 0. Lamarckiana; Oeno- 
thera rnbrinervis from O. Lamarckiana, O. laevifolia, 0. lata, O. oblonga, 
O. nanella, and O. scintillans, etc. Whatever may be the rank assigned to 
these mutations, whether form, variety, or species, there can be no question 
of their polyphyletic origin, nor> in consequence of the connection of 
mutations with variations through such inconstant forms as O. scintillans, 
0. etliptica, and 0. subline ar is , of the possibility of polyphylesis in any 
two distinct though related species or genera. 

KINDS OF INVASION 

282. Continuous and intermittent invasion. With respect to the fre- 
quency of migration, we may distinguish invasion as continuous, or 
intermittent. Continuous invasion, which is indeed usually mutual, occurs 
between contiguous formations of more or less similar character, in which 
there is an annual movement from one into the other, and at the same time 
a forward movement through each, resulting from the invaders established 
the preceding year. By far the greater amount of invasion is of this sort, 
as many readily be seen from the fact that migration varies inversely as the 
distance, and ecesis may decrease even more rapidly than the distance 
increases. The significant feature of continuous invasion is that an outpost 
may be reinforced every year, thus making probable the establishment of new 
outposts from this as a center, and the ultimate extension of the species 
over a wide area. The comparatively short distance and the regular alter- 
nation of migration and ecesis render invasion of this sort very effective. 
An excellent illustration of this is seen in transition areas and regions, which 
are due directly to continuous and usually to mutual invasion. Intermittent 
invasion results commonly from distant carriage, though it may occur very 
rarely between dissimilar adjacent formations, when a temporary swing in 
the physical factors makes ecesis possible for a time. It is characterized by 
the fact that the succession of factors which have brought about the in- 
vasion is more or less accidental and may never recur. Intermittent invasion 
is relatively rare, and from the small number of disseminules affected, it is 
of little importance in modifying vegetation quantitatively. On the other 



INVASION 



235 



hand, since a species may often be carried far from its geographical area, 
it is frequently of great significance in distribution. 

283. Complete and partial invasion. When the movement of invaders 
into a formation is so great that the original occupants arc finally driven out, 
the invasion may be termed complete. Such invasion is found regularly in 
the case of many ruderal formations, and is typical of the later stages of 
many successions. It is ordinarily the result of continuous invasion. If 




Fig. 59. Continuous invasion into a new area; mats of Arenaria 
sajanensis. Silcne acaulis and Sieversia turbinata invading an alpine 
gravel slide. 

the number of invaders is sufficiently small that they may be adopted into 
the formation without radically changing the latter, the invasion is partial 
This is doubtless true- of the greater number of invasions, though these are 
regularly much less striking and important than instances of complete 
invasion. 



284. Permanent and temporary invasion. The permanence of invasion 
depends upon the success attending ecesis, and upon the stability of the 
formation. It has already been noticed that under certain conditions plants 
may germinate and grow, and if they are perennials, even become established, 



236 THE FORMATION 

; and still ecesis be so imperfect that reproduction is impossible. Others may 
rind the conditions sufficiently favorable for propagation, but unfavorable 
for the formation of flowers and fruits. Finally there are plants 
which seem to be perfectly established for a few years, only to dis- 
appear completely. The latter are examples of temporary invasion. It is 
necessary to draw clearly the line between complete and partial invasion in 
this connection. The former is temporary in the initial or intermediate 
stages of nearly all successions, as compared with the ultimate stages, 
though it is in a large degree permanent in comparison with the partial 
invasion of species which are able to maintain themselves for a few years. 
In a sense, there is a real distinction between the two, inasmuch as a par- 
ticular stage of succession is permanent as long as the habitat remains 
essentially the same. A critical study of the species of such stages shows, 
however, that they manifest very different degrees of permanence. Species 
which invade stable vegetation temporarily have been termed adventive by 
A. DeCandolle. Permanent invasion occurs when a species becomes per- 
manently established in a more or less stable formation. It is characteristic 
of the great majority of invaders found in the grassland and forest stages 
of successions. 

Plants which have arisen within a formation or have been a constituent 
part of it since its origin are indigenous. Contrasted with these are the 
species which have invaded the formation since it received its distinctive 
impress : these are derived. The determination of the indigenous and 
derived species of a formation or larger division is of the utmost importance, 
as it enables us to retrace the steps by which the formation has reached its 
present structure, and to reconstruct formations long since disappeared. To 
render it less difficult, it is necessary to scrutinize the derived elements 
closely, first, because it is easiest to recognize the indigenous species by 
eliminating the derived, and second, because this analysis will show that 
not all derived species have entered the formation at the same time and 
from the same sources. Derived species may be termed vicine, when they 
are fully established invaders from adjacent formations or regions, and 
adventitious, when they have come from distant formations and have suc- 
ceeded in establishing themselves. Finally, those derived species which are 
unable to establish themselves permanently are adventive. 

MANNER OF INVASION 

285. Entrance into the habitat. Since the ecesis of invaders depends in 
large measure upon the occupation of the plants in possession, the method 
and degree of invasion will be determined by the presence or absence of 
vegetation. Areas without vegetation are either originally naked or de- 



INVASION 237 

rinded, while vegetation with respect to the degree of occupation is open 
(sporadophytia) , or closed (pycnophytia) . Each type of area presents dif- 
ferent conditions to invaders, largely with respect to the factors determining 
ecesis. Naked habitats, rocks, talus, gravel slides, and dunes, while they 
offer ample opportunity for invasion on account of the lack of occupation, 
are really invaded with the greatest difficulty, not only because they contain 
originally few or no disseminules, but also because of their xerophytic 
character and the difficulty of obtaining a foothold, on account of the ex- 
treme density or instability of the soil. Denuded habitats, blowouts, sand 
draws, ponds, flood plains, wastes, fields, and burns, usually afford maximum 
opportunity for invasion. They invariably contain a large number of dis- 
seminules ready to spring up as soon as the original vegetation is destroyed. 
The surface, moreover, is usually such as to catch disseminules and to offer 
them optimum conditions of moisture and nutrition. Open formations are 
readily invaded, though the increased occupation renders entrance more 
difficult than it is in denuded areas. Closed formations, on the other hand, 
are characterized by a minimum of invasion, partly because invaders from 
different formations find unfavorable conditions in them, but chiefly be- 
cause the occupation of the inhabitants is so complete that invaders are 
unable to establish themselves. 

Invasion takes place by the penetration of single individuals or groups 
of individuals. This will depend in the first place upon the character of 
the disseminule. It is evident that, no matter how numerous the achenes may 
be, the invasion of those anemochorous species with comate or winged seeds 
or one-seeded fruits will be of .the first type, while all species in which the 
disseminule is a several or many-seeded fruit or plant, as in hooked fruits, 
tumble-weeds, etc., will tend to produce a group of invaders. Occasionally 
of course, the accidents of migration will bring together a few one-seeded 
disseminules into a group, or will scatter the seeds of a many-seeded fruit, 
but these constitute relatively rare exceptions. This distinction in the matter 
of invasion is of value in studying the relative rapidity of the latter, and the 
establishment of new centers, but it is of greatest importance in explaining 
the historical arrangement of species in a formation, and hence has a direct 
bearing upon alternation. It is entirely independent of the number of 
invaders, which, as we have seen, depends upon seed-production, mobility, 
distance, occupation, etc., but is based solely upon mode of arrangement, 
and will be found to underlie the primary types of abundance, copious, and 
gregarious. In this connection, it should also be noted that the contingen- 
cies of migration, especially the concomitant action in the same direction of 
two or more distributive agencies, often results in the penetration of a 
group of individuals belonging to two or more species. This may well be 



238 THE FORMATION 

termed mass invasion; it is characteristic of transition areas or regions, and 
along valleys or other natural routes for migration it gives rise to species 
guilds. The movement of species guilds constitutes one of the most com- 
plex and interesting problems in the whole field of invasion, the solution 
of which can be attempted only after the thorough analysis of the simpler 
invasions between formations. A better understanding of the meaning of 
invasion by species guilds is imperative for the natural limitation of regions, 
as at present such groups constitute alien associations in many regions other- 
wise homogeneous. 

286. Influence of levels. The invasion of a formation may occur at three 
different levels: (1) at the level of the facies, (2) below the facies, (3) 
above the facies, depending directly upon the relative height of invaders 
and occupants. The invasion level is an extremely simple matter to deter- 
mine, except in the case of woody plants, such as shrubs and trees, which 
attain their average height only after many years. Its importance is funda- 
mental. The level at which invasion occurs not only determines the 
immediate constitution of the formation, whether its impress shall still be 
given by the occupants or by the invaders or by both together, but it also 
decides the whole future of the formation, i. e., whether the invaders or 
occupants shall persist unmodified or modified. The problem is an extremely 
complex one, but the careful analysis of invasion at each level throws a 
flood of light upon it. The entrance of invaders of the same general height 
as the facies of a formation results regularly in mixed formations. This is 
well illustrated by the structure of the transition areas between two forma- 
tions of the same categary, i. e., forests, meadows, etc. It is seldom, 
however, that the facies and invaders are so equally matched in height and 
other qualities that they remain in equilibrium for a long period. One or 
the other has a slight advantage in height, or the one suffers shading or 
crowding better than the other, is longer-lived or faster-growing, with the 
result that invader yields to occupant, or occupant to invader. It is a well- 
known fact that many mixed formations represent intermediate stages of 
development. 

Invasion at a level different from that of the facies is inevitably followed 
by modification. If the invasion takes place below the facies, the invaders 
will be exterminated gradually, or slowly assimilated. In either case, there 
is little structural change in the formation, and its stability is affected 
slightly or not at all. If the invaders overtop the facies in any considerable 
number, the entire formation undergoes partial or complete modification, 
or in extreme cases it disappears, as is typically the case in succession. A 
peculiar variation of invasion at a level above the facies is seen where woody 



SUCCESSION 239 

plants invade grassland, when the trees or shrubs become more or less uni- 
formly scattered in an open woodland or open thicket. Here the grassland 
takes on an altogether different appearance superficially, though it is usually 
unchanged, except beneath and about the invaders, where either adaptation 
or extermination results. Finally, it should be borne in mind that the inva- 
sion of a particular formation, especially in the case of layered thickets and 
forests, often takes place at two levels, at the height of the facies and below 
the facies. 

INVESTIGATION OF INVASION 

287. The methods to be used in the study of invasion are those already 
described elsewhere. The migration circle is of the first importance because 
it makes it possible to secure an accurate record of actual movement. Quad- 
rat and transect are valuable, but from their nature they are more service- 
able for ecesis than for migration. All of these should be of the permanent 
type, in order that the fate of invaders may be followed for several years at 
least. Permanent areas furnish evidence of the changes wrought in the 
actual vegetation, while denuded ones can serve only to show the potential 
migration and ecesis of the constituent species. Transition zones and areas 
are special seats of invasion ; they are best studied by means of the belt tran- 
sect and the ecotone chart. The movement of a line of invaders or of scat- 
tered outposts is traced by the use of labeled stakes at the points concerned. 
It is clear that this method will yield conclusive data in regard to the great 
invasions between regions, such as the movement of species guilds, the ad- 
vance of the forest frontier, etc. When invasion is scattered, factor instru- 
ments can not be used to advantage, but where the invading line is well 
marked, or where extra-formational areas occur, a knowledge of the phys- 
ical factors is a great aid. 

An invasion that has been completed can not be studied in the manner in- 
dicated. A method of comparison must be used, in order to determine the 
original home of the invaders. For this an exact knowledge of the contig- 
uous formations and of the abundance of the species common to all is a pre- 
requisite. With this as a basis, it is usually a simple task to refer all the 
species of the formation concerned to their proper place in the groups, indig- 
enous, derived, and adventitious. 

SUCCESSION 

288. Concept. Succession is the phenomenon in which a series of inva- 
sions occurs in the same spot. It is important, however, to distinguish clearly 
between succession and invasion, for, while, the one is the direct result of the 



24O THE FORMATION 

other, not all invasion produces succession. The number of invaders must be 
large enough, or their effect must be sufficiently modifying or controlling to 
bring about the gradual decrease or disappearance of the original occupants, 
or a succession will not be established. Partial or temporary invasion can 
never initiate a succession unless the reaction of the invaders upon the habitat 
is very great. Complete and permanent invasion, on the other hand, regularly 
produces successions, except in the rare cases where a stable formation en- 
tirely replaces a less stable one without the intervention of other stages. 
vSuccession depends in the first degree upon invasion in such quantity and of 
such character that the reaction of the invaders upon the habitat will prepare 
the way for further invasion. The characteristic presence of stages in a 
succession, which normally correspond to formations, is due to the peculiar 
operation of invasion with reaction. In the case of a denuded habitat, for 
example, migration from adjacent formations is constantly taking place, but 
only a small number of migrants, especially adapted to somewhat extreme 
conditions, are able to become established in it. These reach a maximum 
development in size or number, and in so doing react upon the habitat in 
such a way that more and more of the dormant disseminules present, as well 
as those constantly coining into it, find the conditions favorable for germina- 
tion and growth. The latter, as they in turn attain their maximum, cause 
the gradual disappearance of the species of the first stage, and at the same 
time prepare the way for the individuals of the succeeding formation. It is 
at present impossible to determine to what degree this substitution is due to 
the struggle for existence between the individuals of each species and be- 
tween the somewhat similar species of each stage, and to what degree it 
arises out of the physical reaction. 

It is evident that geological succession is but a larger expression of the 
same phenomenon, dealing with infinitely greater periods of time, and pro- 
duced by physical changes of such intensity as to give each geological stage 
its peculiar stamp. If, however, the geological record were sufficiently com- 
plete, we should find unquestionably that these great successions merely rep- 
resent the stable termini of many series of smaller changes, such as are 
found everywhere in recent or existing vegetation. 

289. Kinds of succession. The fundamental causes of succession are in- 
vasion and reaction, but the initial causes of a particular succession are to 
be sought in the physical or biological disturbances of a habitat or forma- 
tion. With reference to the initial cause, we may distinguish normal succes- 
sion, which begins with nudation, and ends in stabilization, and anomalous 
succession, in which the fades of an ultimate stage of a normal succession are 
replaced by other species, or in which the direction of movement is radically 



SUCCESSION" 241 

changed. The former is of universal occurrence and recurrence ; the latter 
operates upon relatively few ultimate formations. In the origin of normal 
successions, nudation may be brought about by the production of new soils 
or habitats, or by the destruction of the formation which already occupies a 
habitat. In a few cases, the way in which the habitat arises or becomes de- 
nuded is not decisive as to the vegetation that is developed upon it, but as a 
rule the cause of nudation plays as important a part, in the development of 
a succession as does the reaction exerted by the invaders. The importance 
of this fact has been insisted upon under invasion. New soils present ex- 
treme conditions for ecesis, possess few or no dormant disseminules, and in 
consequence their successions take place slowly and exhibit many stages. 
Denuded soils as a rule offer optimum conditions for ecesis as a result of 
the action of the previous succession, dormant seeds and propagules are 
abundant, and the revegetation of such habitats takes place rapidly and 
shows few stages. The former may be termed primary succession, the latter 
secondary succession. 

PRIMARY SUCCESSIONS 

290. These arise on newly formed soils, or upon surfaces exposed for the 
first time, which have in consequence never borne vegetation before. In 
general they are characteristic of mountain regions, where weathering is the 
rule, and of lowlands and shores, where sedimentation or elevation con- 
stantly occur. The principal physical phenomena which bring about the 
formation of new soils are: (1) elevation, (2) volcanic action, (3) weather- 
ing, with or without transport. 

291. Succession through elevation. Elevation was of very frequent oc- 
currence during the earlier, more plastic conditions of the earth, and the suc- 
cessions arising as a result of it must have been important features of the 
vegetation of geological periods. To-day, elevation is of much less impor- 
tance in changing physiography, and its operation is confined to volcanic 
islands, coral reefs, and islets, and to rare movements or displacements in 
seacoasts, lake beds, shore lines, etc. There lias been no investigation of the 
development of vegetation on islands that are rising, or have recently been 
elevated, probably because of the slow growth of coral reefs and the rare 
appearance of volcanic islands. On coral reefs, the first vegetation is in- 
variably marine, but as the reef rises higher above the surf line and the tide, 
the vegetation passes into a xerophytic terrestrial type adapted to an imper- 
vious rock soil ; and ultimately becomes mesophytic. In volcanic islands, 
unless they are mere rocks over which the waves rush, the succession must 
always begin with a xerophytic rock formation. The best known example 
of a rising coast line is found in Norway and Sweden, where the southeast- 



242 



THE FORMATION 



ern coast is rising at the rate of five or six feet a century. There can be little 
question that such changes of level will produce marked changes in vegeta- 
tion, but the modification will be so gradual as to be scarcely perceptible in a 
single generation. It is probable that the forests of the Atlantic coastal 
plains are the ultimate stages of successions initiated at the time of the final 
elevation of the sea bottom along the coast line. 




Fig. 60. A lichen formation (Lecanord-Physcia-petvium), the first 
stage of the typical primary succession (Lecanora-Picea-sphyrium) of 
the Colorado mountains. 

292. Succession through volcanic action. The deposition of volcanic 
ashes and flows of lava are relatively infrequent at present, occurring only 
in the immediate vicinity of active volcanoes, chiefly in or near the tropics. 
Successions of this sort are in consequence not only rare, but they are also 
relatively inaccessible to investigators. They have been studied in a few 
cases, for example, those of Krakatoa by Treub, but this study has been con- 
fined to the general features of revegetation. Ash fields and lava beds are 
widely different in compactness, but they agree in having a low water- and 
nutrition-content. The pioneer plants in both will be intense xerophytes, but 
the soil differences will determine that these shall be sandbinders in the for- 
mer, and rock-weathering plants in the latter. 






SUCCESSION 243 

/ 

293. Weathering. Practically all primary successions start on soils pro- 
duced by weathering. This is also true of coral or volcanic islets and of lava 
beds, for no terrestrial vegetation can secure a foothold upon them until the 
surface of the rock has been to some extent decomposed or disintegrated. 
Weathering, as is well known, consists of two processes, disintegration and 
decomposition, which usually operate successively, though they are some- 
times concomitant. Disintegration usually precedes, especially in rock 
masses, and unless it is soon followed by decomposition, results in dysgeo- 
genous soils. Decomposition often goes hand in hand with disintegration, 
or it takes place so rapidly and perfectly that it alone seems to be present. 
In either case, the resulting soil is eugeogenous. The relation of decompo- 
sition to disintegration determines the size and compactness of the soil par- 
ticles, and upon the latter depend the porosity, capillarity, and hygrosco- 
picity of the soil. These control iu large degree the character of the first 
vegetation to appear on the soil. 

Another point of -fundamental value in determining revegetation is the 
disposition of the weathered rock. If it remains in situ, it will evidently 
differ in respect to compactness, homogeneity, nutrition-content, water-con- 
tent, disseminules, etc., from weathered material which has been transported. 
An essential difference also arises from the fact that a rock may be weath- 
ered a long distance from the place where the decomposed particles are finally 
deposited, and in the midst of a vegetation very different from that found 
in the region of deposit. The disposition of the weathered material affords 
in consequence a satisfactory basis for the arrangement of primary succes- 
sions. The following classification is proposed, based upon the soil groups 
established by Merrill. 1 

294. Succession in residuary soils. Residuary soils are always sedentary, 
i. e., they are formed in situ. They show certain differences dependent upon 
the rock from which they originate, which may be mixed crystalline shale, 
sandstone, or limestone, but the thoroughness of decomposition causes these 
differences to be comparatively small. Residuary soils are typically eugeo- 
genous; their successions in consequence usually begin with mesophytes, and 
consist of a few stages. The soluble salt-content is comparatively low, since 
all soluble matters are readily leached out. Successions in these soils are 
especially characteristic of shale, sandstone, and limestone ledges or banks. 
Cumulose deposits, like residuary ones, are sedentary in character, but as 
they are produced by the accumulation of organic matter, they will be con- 
sidered under reactions of vegetation upon habitat. 

1 Rocks, Rock-weathering, and Soils, BOO. 1897. 



244 



THE FORMATION 



295. Succession in colluvial soils. Colluvial deposits owe their aggrega- 
tion solely or chiefly to the action of gravity. They are the immediate result 
of the disintegration of cliffs, ledges, and mountain sides, decomposition ap- 
pearing later as a secondary factor. The masses and particles arising from 
disintegration are extremely variable in size, but they agree as a rule in their 
angular shape. The typical example of the colluvial deposit is the talus, 
which may originate from any kind of rock, and contains pieces of all sizes. 
Gravel slides differ from ordinary talus in being composed of more uniform 
particles, which are worn round by slipping down the slope in response to 
gravity and surface wash. Boulder fields are to be regarded as talus pro- 




Fig. 61. Talus arising from the disintegration of a granitic cliff; the 
rocks are covered with crustose lichens. 



duced by weathering under the influence of joints, resulting in huge boul- 
ders which become more and more rounded under the action of water and 
gravity. This statement applies to those fields which are in connection with 
some cliff that is weathering in this fashion ; otherwise, boulder fields are of 
aqueous or glacial origin. The character of the successions in talus will 
depend upon the kind of rock in the latter. If the rock is igneous or meta- 
morphic, decomposition will be slow, and the soil will be dysgeogenous. 
Successions on such talus consist of many stages, and the formations are for 
a long time open and xerophytic. In talus formed from sedimentary rocks, 



SUCCESSION 245 

especially shales, limestones, and calcareous sandstones, decomposition is 
much more rapid, and the successions are simpler and more mesophytic. 

296. Succession in alluvial soils. Alluvial soils are fluvial when laid down 
by streams and rivers, and litoral when washed up by the waves or tides. 
They are formed when any obstacle retards the movement of the water, de- 
creasing its carrying power, and causing the deposit of part or all of its load. 
They consist of more or less rounded, finely comminuted particles, mingled 
witli organic matter and detritus. Alluvial deposits are especially frequent 
at the mouth of streams and rivers, on their terraces and flood plains, and 
along silting banks as compared with the erosion banks of meanders. The 
filling of ponds by the erosion clue to surface drainage, and of lakes by the 
deposition of the loads of streams that enter them, results in the formation 
of new alluvium. A similar phenomenon occurs along coasts, where bays 
and inlets are slowly converted into marshes in consequence of being shal- 
lowed by the material washed in by the waves and tides. Such paludal de- 
posits are invariably salt water or brackish. Contrasted with these, which 
are uniformly black in consequence of the large amount of organic matter 
present, are the sandbars and beaches, which, though due to the same agents, 
are light grey or white in color, because of the constant leaching by the 
waves. Two kinds of alluvial deposits may accordingly be distinguished: 
(1) those black with organic matter, and little disturbed by water, and (2) 
those of a light color, which are constantly swept by the waves. The suc- 
cessions corresponding to these are radically different. In the first, the pio- 
neer vegetation is hydrophytic, consisting largely of amphibious plants. 
The pioneer stages retard the movement of the water more and more, and 
correspondingly hasten the deposition of its load. The marsh bed slowly 
rises in consequence, and finally the marsh begins to dry out, passing first 
into a wet meadow, and then into a meadow of the normal type. A notable 
exception to this sequence occurs when the swamp contains organic matter 
or salts in excess, in which case the vegetation consists indefinitely of swamp 
xerophvtes, or halophytes. The first vegetation on fresh water sandbars is 
xerophytic, or, properly, dissophytic, unless they remain water-swept, and 
the ultimate stages of their successions are mesophytic woodlands composed 
of water-loving genera, Populus, Salix, etc. It seems certain, however, that 
these will finally give way to longer-lived hardwoods. Maritime sandbars 
and beaches are always saline, and their successions run their short course 
of development entirely within the group of halophytes, unless the retreat 
of the sea or fresh-water floods change the character of the soil. The chem- 
ical action of underground waters also produces new soils, which might be 
classed as alluvial. These soils are essentially rock deposits, travertine, sili- 



246 



THE FORMATION 



cious sinter, etc., made by iron and lime springs and by geysers, and they 
must be changed by decomposition into soils proper to be comparable with 
alluvial soils. 

297. Succession in aeolian soils. The only wind-borne soils of geological 
importance at the present time are those which form dunes, both inland and 
coastal. Aeolian deposits consist largely of rounded sand particles, which 
are*of almost uniform size in any particular dune, but vary greatly in dunes 
of different ages. The reaction of the pioneers on dunes plays an important 




Fig. 62. Talus arising from the decomposition of granite; the gravel 
is covered with a formation of foliose lichens (Parmelia-chalicium) , 
the second stage of the primary talus succession; the herbs are pioneers 
of the next stage. 



part in building the latter, but the immense dunes of inland deserts, which 
are entirely destitute of vegetation, seem to indicate that its value has been 
overestimated. The first stages in dune successions are dissophytic, i. e., the 
plants grow in a soil of medium or high water-content, but in an atmosphere 
that is extremely xerophytic. The ultimate stages vary widely in accord- 
ance with the region in which they occur ; they may be xerophytic heaths or 
mesophytic meadows and forests. Because of their striking character and 
economic significance, dunes have received much attention, with the result 



SUCCESSION 247 

lhat their successions are the most thoroughly known of all. Prairie and 
steppe formations are probably to be regarded as the ultimate stages of suc- 
cessions established on wind-borne loess, and it is possible that the same is 
true of sand-hill vegetation in the prairie province. 

298. Succession in glacial soils. The formation of glacial deposits is at 
present confined to alpine and arctic regions. Recent successions in such 
soils are localized in these regions, and are in consequence relatively unim- 
portant. There can be little question, however, that the thorough investiga- 
tion of succession in and near the moraines of existing glaciers will throw 
much light upon the successions of the glacial period. Moraines, drumlins, 
eskars, and alluvial cones represent the various kinds of glacial deposits. 
They agree in being heterogeneous in composition, and are covered to-day 
with ultimate stages of vegetation, except in the immediate vicinity of 
glaciers. 

SECONDARY SU2CES310NS 

299. Generally speaking, all successions on denuded soils are secondary. 
When vegetation is completely removed by excessive erosion, it is an open 
question whether the resulting habitat is to be regarded as new or denuded. 
Erosion is rarely so extreme and so rapid, however, as to produce such a 
condition, even when it results from cultivation or deforestation. It is, more- 
over, especially characteristic of newly formed soils, and in studying succes- 
sion in eroded habitats, it is fundamentally important to determine whether 
erosion has produced denudation, or has operated upon a new soil. The 
great majority of secondary successions owe their origin to floods, animals, 
or the activities of man, and they agree in occurring upon decomposed soils 
of medium water-content, which contain considerable organic matter, and a 
large number of dormant migrants. These successions consist of relatively 
few stages, and are rarely of extreme character. 

300. Succession in eroded soils. Eroded soils show considerable differ- 
ences, as they arise in consequence of erosion by water or by wind, though 
the initial stages of revegetation derive their character more from the aggre- 
gation of the soil than from the nature of the erosive agent. Eroded soils 
are as a rule xerophytic. In the case of erosion by water, dysgeogenous soils 
are readily worn away in consequence of their lack of cohesion, as in sand- 
draws, etc., while eugeogenous soils are easily eroded only on slopes, as in 
the case of ravines, hillsides, etc. In the former, the extreme porosity and 
slight capillarity of the sand and gravel result in a low water-content. In 
the finer soils, the water-content is also low, on account of the excessive run- 



248 THE FORMATION 

off, due to compactness of the particles and to the slope. The erosive action 
of winds upon soils bearing vegetation is not very general ; it is found to 
some extent in more or less established dunes, and exists in a marked degree 
in buttes, mushroom rocks, and blowouts. The first two are regularly xero- 
phytic, the last as a rule, dissophytic. The early stages of successions in 
eroded soils are composed of xerophytes. In loose soils, these are forms 
capable of binding the soil particles together, thus preventing wash, and in- 
creasing the accumulation of fine particles, especially of organic matter. In 
compact soils, the effect is much the same; the pioneers not only decrease 
erosion, but at the same time also increase the water-content by retarding the 
movement of the run-off. 

301. Succession in flooded soils. The universal response of vegetation to 
floods is found in the amphibious plant, which is a plastic form capable of 
adjustment to very different water-contents. Floods are confined largely to 
river basins and coasts. In hilly and mountainous regions, where the slope 
is great, any considerable accumulation of flood waters is now impossible, 
although of frequent occurrence when land forms were more plastic. 

In all streams that have become graded, the fall is insufficient to carry off 
the surplus water in the spring when snows are melting rapidly, or at times 
of unusual precipitation. These waters accumulate, and, overflowing the 
banks, spread out over the lowlands, resulting in the formation of a well- 
defined flood plain. This is a periodical occurrence with mature streams, 
and it occurs more or less regularly with all that are not torrent-like in char- 
acter. The effect of the overflow is to destroy or to place at a disadvantage 
those plants of the flood plain that are not hydrophytes. At the same time, 
a thin layer of fresh silt is deposited upon the valley floor of sand or allu- 
vium. Flooding is most frequent and of longest duration near the banks of 
the stream. It extends more or less uniformly over the flood plain, and dis- 
appears gradually or abruptly as the latter rises into the bench above. Floods 
destroy vegetation and make a place for secondary successions by drowning 
out mesophytic species, by washing away the aquatic forms of ponds' and 
pools, and by the erosion of banks and sandbars. They affect the amphib- 
ious vegetation of swamp and shore to a certain extent, but, unless the period 
of flooding is long, they tend to emphasize such formations rather than to 
destroy them. The still-water formations of many cutoff and oxbow lakes 
owe their origin to a river which cuts across a meander in time of flood. 
This result is more often attained by the alternate silting and erosion of a 
meandering river by which it cuts across a bend in its channel. The usual 
successions in flooded lands are short as a rule ; amphibious algae, liverworts, 
and mosses soon give way to ruderal plants, and these in turn to the original 



SUCCESSION 249 

mesophytes of meadows, or dissophytes of sandbars. In the case of ponds 
and pools, the process of washing-out or silting-up merely removes or de- 
stroys the vegetation, without effectively modifying the habitat, and the sec- 
ondary successions that follow are extremely short. 

302. Succession by subsidence. Subsidence is a factor of the most pro- 
found importance in changing vegetation. It operates over vast areas 
through immense periods of time. For these reasons, the changes are so 
slow as to be almost imperceptible, and the resulting successions can be 
studied only in the geological record. Extensive subsidence is confined to- 
day to coastal plains, as in Greenland, the south Atlantic coast, and the region 
of the Mississippi delta, Avhere its effects are merged with the paludation of 
tidal rivers, and the wave and tide erosion of the sea shore. Such succes- 
sions are unique, inasmuch as the denuding force operates very slowly in- 
stead of quickly, and the first pioneers of the new vegetation appear before 
the original formation has been destroyed. In all cases, the succession is 
from mesophytic or halophytic formations to paludose, and, finally, marine 
vegetation. In small areas of subsidence, such as shore slips along lakes 
and streams, sink holes, and sunken bogs, the succession is usually both short 
and simple, mesophytes giving place to amphibious and ultimately to aquatic 
forms. 

303. Successions in landslips. Landslips occur only in montane and hilly 
regions, and here they are merely of local importance. In many respects, 
they are not unlike talus ; they show* essential differences, however, in that 
they are not sorted by gravity, and in that they destroy vegetation almost 
instantly. The succession arises as a rule, not upon the original soil, but 
upon that of the landslip, and, as pointed out elsewhere, might well be re- 
garded as primary. 

304. Succession in drained, or dried soils. In geological times, the subsi- 
dence of barriers must often have produced drainage and drying-out, just as 
elevation frequently resulted in flooding and lake formation. At the present 
time, the drying-out of lakes and ponds is the result of artificial drainage, or 
of climatic changes. The former will be considered under successions 
brought about by the agency of man. Climatic changes when general oper- 
ate so slowly that the stages of such successions are perceptible only when 
recorded in strata. More locally, climate swings back and forth through a 
period of years, with the result that in dry years the swamps and ponds of 
wetter seasons are dried out, and the vegetation destroyed or changed. If 
the process be gradual, the succession passes from hydrophytic through am- 



250 



THE FORMATION 



phibious to mesophytic, and, in dry regions, xerophytic conditions. When 
the process of drying out occurs rapidly, as in a single summer, the original 
formation is destroyed, and the new vegetation consists largely of ruderal 
plants. A peculiar effect of climate occurs in regions with poor drainage, 
where the result of intense evaporation is to produce alkaline basins and salt 
lakes, in which the succession becomes more and more open, and is finally 
represented by a few stabilized halophytes, or disappears completely. 











Fig. 63. A typical gravel slide (talus) of the Rocky mountains, 
before invasion. 

305. Succession by animal agency. Successions of this class are alto- 
gether of secondary importance, the instances in which animals produce de- 
nudation being relatively few. Such are the heaps of dirt thrown up by 
prairie dogs and other burrowing animals, upon which ruderal plants are 
first established, to be finally crowded out by the species of the original for- 
mation. Buffalo wallows furnish examples of similar successions in which 
the initial stages are subruderal, while overstocking and overgrazing fre- 
quently produce the same result with ruderal plants. 



306. Succession by human agency. The activities of man in changing 
the surface of the earth arc so diverse that it is impossible to fit the resulting 
successions in a natural system. While man does not exactly make new 
soils, he exposes soils in various operations : mining, irrigation, railroad 



SUCCESSION 25I 

building, etc. He destroys vegetation by fires, lumbering, cultivation, and 
drainage, and if he can not control climate, he at least modifies its natural 
effects by irrigation and the conservation of moisture. The operations of 
man extend from seacoasts and swampy lowlands through mesophytic 
forests and prairies to the driest uplands and inlands. Since the adjacent 
formations determine in large degree the course and constitution of a succes- 
sion, it will be seen that the effects of any particular activity upon vegeta- 
tion will differ greatly in different regions. For convenience, all classes of 
successions arising from the presence and activity of man w 7 ill be considered 
in this place, though, as indicated above, some might well be regarded as 
producing- primary successions, while others produce anomalous ones. 

307. Succession in burned areas. It will suffice merely to point out that 
"burns" may arise naturally through lightning, volcanic cinders, lava flows, 
etc., but the chances are so slight that these causes may be ignored. The 
causes of fires are legion, and as they have little or no effect upon results, 
they need not be considered. From their nature, fires are of little signifi- 
cance in open vegetation, deserts, polar barrens, alpine fields, etc., since the 
area of the burn can never be large. In closed formations, the extent of 
fires is limited only by the area of the vegetation, and the effect of wind, 
rain, and other forces. Forest fires usually occur during the resting period, 
except in the case of coniferous forests. In grassland, the living parts are 
underground during autumn and winter, when prairie fires commonly occur. 
As a consequence, the repeated annual burning of meadow or prairie does 
not result in denudation and subsequent succession. On the contrary, it 
acts in part as a stabilizing agent, inasmuch as it injures the typical vegeta- 
tion forms of grassland much less than it does the woody invaders. All for- 
mations with perennial parts above ground, viz., thicket, open woodland, 
and forest, are seriously injured by fire. A severe general fire destroys the 
vegetation completely ; a local fire destroys the formation in restricted areas ; 
while a slight or superficial burn removes the undergrowth and hastens the 
disappearance of the weaker trees. In the latter case, while the primary 
layer of the forest remains the same, succession takes place in the herbaceous 
and shrubbv layers. These successions are peculiar in that they are com- 
posed almost wholly of the proper species of the forest, and that they are 
very short, showing only a few poorly denned stages. A local fire initiates 
a succession in which the pioneers are derived largely from the original for- 
mation, particularly when the latter encloses the burned area more or less 
completely. The constitution of the intermediate and ultimate stages will 
depend in a larger degree still upon the size and position of the burn. When 
a particular formation is destroyed wholly or in large part, the first stages 
of the new vegetation are made up by invaders from the adjacent formations. 



252 



THE FORMATION 



In the most perfect types of succession, this dissimilarity between the new 
and the old vegetation continues to the last stage, in which the reappearance 
of the facies precedes that of the subordinate layers. In many forest suc- 
cessions, however, the general physical similarity of the ultimate stages per- 
mits the early reappearance of the herbaceous and shrubby species, and the 
final stages affect the facies alone. Successions in burned areas operate usu- 
ally within the water-content groups. The reconstruction of a mesophytic 
forest takes place by means of mesophytes ; of the rarer xerophytic and hy- 
drophytic forests, through xerophytes and hydrophytes respectively. This 
is due to the fact that the alteration of the soil is slight, except where the 
burning of the vegetation permits the entrance of erosion, as on mountain 
slopes. 




Fig. 64. Gravel slide formation (Pseudocymopterus-Mentzelia-chal- 
icium), stage III of the talus succession. 

308. Succession, in lumbered areas. Commercial lumbering, especially 
where practiced for wood-pulp as well as for timber, results in complete or 
nearly complete destruction of the vegetation by removal and the change 
from diffuse light to sunlight, or by the action of erosion upon the exposed 
surface. In the first place, short mesophytic successions will result; in the 
second, the successions will be long and complex, passing through decreas- 
mgly xerophytic conditions to a stable mesophytic forest. Where a forest 



SUCCESSION 253 

is cut over for certain species alone, the undisturbed trees soon take full pos- 
session, though the causes effective in the beginning will ultimately restore 
the original fades in many instances. Such successions are anomalous, and 
will be treated under that head. 

309. Succession by cultivation. The clearing of forests and the "break- 
ing" of grassland for cultivation destroy the original vegetation ; the tempo- 
rary or permanent abandonment of cultivated fields then permits the entrance 
of ruderal species, which are the pioneers of new successions. This phe- 
nomenon takes place annually in fields after harvest, resulting in the second- 
ary formations of Warming, in which practically the same species reappear 
year after year: In fields that lie fallow for several years, or are perma- 
nentlv abandoned, the first ruderal plants are displaced by newcomers, or 
certain of them become dominant at the expense of others. In a few years, 
these are crowded out by invaders from the adjacent formations, and the 
field is ultimately reclaimed by the original vegetation, unless this has en- 
tirely disappeared from the region. The number of stages depends chiefly 
upon whether the final formation is to be grassland or woodland. Other 
activities of man, such as the construction of buildings, roads, railways, 
canals, etc., remove the native vegetation, and make room for the rapid de- 
velopment of ruderal formations. In and about cities, where the original 
formations have entirely disappeared, the chance for succession is remote, 
and the initial ruderal stages become more or less stabilized. Elsewhere 
the usual successions are established, and the ruderal formation finally gives 
way to the dominant type. In mountain and desert regions, where ruderal 
plants are rare or lacking, their place is taken by subruderal forms, species 
of the native vegetation capable of rapid movement in them. These, like 
ruderal plants, are gradually replaced by other native species of less mobil- 
ity, but of greater persistence, resulting in a short succession operating often 
within a single formation. From the nature of cultivated plants, succession 
after cultivation generally operates within the mesophytic series. 

310. Succession by drainage. Successions of this kind show much the 
same stages as are found in those due to flooding. They proceed from 
aquatic or swamp formations to mesophytic termini, either grassland or 
woodland. When drainage takes place rapidly and completely, the pioneer 
stages are usually xerophytic; cases of this sort, however, are infrequent. 

311. Succession by irrigation. Irrigation produces short successions of 
peculiar stamp along the courses of irrigating canals and ditches, and in the 
vicinity of reservoirs. These are recent, as a rule, and are usually found in 



254 THE FORMATION 

the midst of cultivated lands, so that their complete history is still a matter 
of conjecture. The original xerophytes are forced out not only by the dis- 
turbance of the soil, but also by its increased water-content. A few of them 
often thrive under the new conditions, and, together with the usual ruderal 
plants and a large number of lowland mesophytes and amphibious forms 
derived from the banks of the parent stream, constitute a heterogeneous as- 
sociation. This is doubtless to be regarded as an initial stage of a succes- 
sion, but it is an open question whether the succession will early be stabilized 
as a new formation, or whether the original vegetation will sooner or later 
be reestablished under somewhat mesophytic conditions. From the number 
of mesophytes and from the behavior of valleys, it seems certain that the 
banks of such canals will ultimately be occupied by a formation more meso- 
phytic than hydrophytic, into which some of the surrounding xerophytes of 
plastic nature have been adopted. 

312. Anomalous successions are those in which the physical change in the 
habitat is relatively slight, resulting in a displacement of the ultimate stage, 
or the disturbance of the usual sequence, merely, instead of the destruction 
and reconstruction of a formation, or the gradual development of a new 
series of stages on new soil. In nature, the ultimate grass or forest stage 
of a normal succession is often replaced .by a similar formation, especially 
if the facies be few or single. It is evident that certain trees naturally re- 
place others in the last stages of a forest succession, without making the lat- 
ter anomalous. The last occurs only when a normal stage is replaced by 
one belonging properly to an entirely different succession, as when a conif- 
erous forest replaces a deciduous one in a hardwood region. The presence 
and development of such successions can be determined only after the nor- 
mal types are known. The interpolation of a foreign stage in a natural suc- 
cession, or a change of direction, by which a succession that is mesotropic 
again becomes hydrophytic, is easily explained when it is the result of arti- 
ficial agents, as is often the case. In nature, anomalous successions are 
commonly the result of a slow backward and forward swing of climatic 
conditions. 

313. Perfect and imperfect successions. A normal succession will regu- 
larly be perfect ; it passes in the usual sequence from initial to ultimate con- 
ditions without interruption or omission. Imperfect succession results when 
one or more of the ordinary stages is omitted anywhere in the course, and a 
later stage appears before its turn. It will occur at any time when a new or 
denuded habitat becomes so surrounded by other vegetation that the forma- 
tions which usually furnish the next invaders are unable to do so, or when 



SUCCESSION 



255 



the abundance and mobility of certain species enable them to take possession 
before their proper turn, and to the exclusion of the regular stage. Incom- 
plete successions are of great significance, inasmuch as they indicate that the 
stages of a succession are often due more to biological than to physical causes, 
the proximity and mobility of the adjacent species being more determinative 
than the physical factors. Subalpine gravel slides regularly pass through 
the rosette, mat. turf, thicket, woodland, and forest stages; occasionally, 
however, they pass immediately from the rosette, or mat condition, to an 
aspen thicket which represents the next to the last stage. Such successions 
are by nc means infrequent in hilly and montane regions ; in regions phys- 
iographically more mature or stable, perfect successions are almost invari- 
ablv the rule. 




Fig. 55. Half gravel slide formation (Elymus-Muhlenbergia-chal- 
icium), stage IV of the talus succession. 



314. Stabilization. It may be stated as a general principle that vegetation 
moves constantly and gradually toward stabilization. Each successive stage 
modifies the physical factors, and dominates the habitat more and more, in 
such a way that the latter seems to respond to the formation rather than this 
to the habitat. The more advanced the succession, i. e., the degree of sta- 
bilization, the greater the climatic or physiographic change necessary to 
disturb it, with the result that such disturbances are much more frequent in 



256 THE FORMATION 

the earlier stages than in the later development. Constant, gradual move- 
ment toward a stable formation is characteristic of continuous succession. 
Contrasted with this is intermittent succession, in which the succession 
swings for a time in one direction, from xerophytic to mesophytic for ex- 
ample, and then moves in the opposite direction, often passing through the 
same stages. This phenomenon usually is characteristic only of the less 
stable stages, and is generally produced by a climatic swing, in which a series 
of hot or dry years is followed by one of cold or wet years, or the reverse. 
The same effect upon a vast scale is produced by alternate elevation and 
subsidence, but these operate through such great periods of time that one 
can not trace, but can only conjecture their effects. A normal continuous 
succession frequently changes its direction of movement, or its type, in trans- 
ition regions or in areas where the outposts of a new flora are rapidly ad- 
vancing, as in wide mesophytic valleys that run down into or traverse plains. 
Here the change is often sudden, and grass and desert formations are 
replaced by thickets and forests, resulting in abrupt succession. Species 
guilds are typical examples of this. More rarely, a stage foreign to the 
succession will be interpolated, replacing a normal stage, or slipping in be- 
tween two such, though finally disappearing before the next regular forma- 
tion. This may be distinguished as interpolated succession. 

The apparent terminus of all stabilization is the forest, on account of the 
thoroughness with which it controls the habitat. A close examination of 
vegetation, however, will show that its stable terms are dependent in the first 
degree upon the character of the region in which the formation is indigenous. 
It is obviously impossible that successions in desert lands, in polar barrens, 
or upon alpine stretches should terminate in forest stages. In these, grass- 
land must be the ultimate condition, except in those extreme habitats, alpine 
and polar, where mosses and lichens represent the highest type of existing 
vegetation. Forests are ultimate for all successions in habitats belonging to 
a region generally wooded, while grassland represents the terminus of prairie 
and plains successions as well as of many arctic-alpine ones. 

CAUSES AND REACTIONS 

315. The initial cause of a succession must be sought in a physical change 
in the habitat; its continuance depends upon the reaction which each stage 
of vegetation exerts upon the physical factors which constitute the habitat. 
A single exception to this is found in anomalous successions, where the 
change of formation often hinges upon the appearance of remote or foreign 
disseminules. The causes which initiate successions have already been con- 
sidered; they may be summarized as follows: (1) weathering, (2) erosion, 
(3) elevation, (4) subsidence, (5) climatic changes, (6) artificial changes. 



SUCCESSION 257 

The effect of succeeding stages of vegetation upon a new or denuded habitat 
usually finds expression in a change of the habitat with respect to a particu- 
lar factor, and in a definite direction. Often, there is a primary reaction, 
and one or more secondary ones, which are corollaries of it. Rarely, there 
are two or more coordinate reactions. The general ways in which vegeta- 
tion reacts upon the habitat are the following: (1) by preventing weather- 
ing, (2) by binding aeolian soils, (3) by reducing run-off and preventing 
erosion, (4) by filling with silt and plant remains, (5) by enriching the 
.soil, (6) by exhausting the soi', (7) by accumulating humus, (8) by modi- 
fying atmospheric factors. The direction of the movement of a succession 
is the immediate result of its reaction. From the fundamental nature of 
vegetation, it must be expressed in terms of water-content. The reaction is 
often so great that the habitat undergoes a profound change in the course 
of the succession, changing from hydrophytic to mesophytic or xerophytic, 
or the reverse. This is characteristic of newly formed or exposed soils. 
Such successions are xerotropic, mcsotropic, or hydrotropic, according to 
the ultimate condition of the habitat. When the reaction is less marked, 
the type of habitat does not change materially, and the successions are xero- 
static, mesostat'iCj or hydrostatic, depending upon the water-content. Such 
conditions obtain for the most part only in denuded habitats. 

316. Succession by preventing weathering. Reactions of this nature 
occur especially in alpine and boreal regions, in the earlier stages of lichen- 
moss successions. They are typical of igneous and metamorphic rocks in 
which disintegration regularly precedes decomposition. The influence of the 
vegetation is best seen in the lichen stages, where the crustose forms make a 
compact layer, which diminishes the effect of the atmospheric factors pro- 
ducing disintegration. In alpine regions especially, this protection is so per- 
fect that the crustose lichens may almost be regarded as the last stage of a 
succession. There are no recorded observations which bear upon this point, 
but it seems certain that the pioneer rock lichens, Lccanora, Lecidea, Biatora, 
Buellia, and Acarospora, cover alpine rocks for decades, if not for centuries. 
Ultimately, however, the slow decomposition of the rock surface beneath the 
thallus has its effect. Tiny furrows and pockets are formed, in which water 
accumulates to carry on its ceaseless work, and the compact crustose cover- 
ing is finally ruptured, permitting the entrance of foliose forms. The latter, 
like the mosses, doubtless protect rock surfaces, especially those of the softer 
rocks, in a slight degree against the influence of weathering, but this is more 
than offset by their activity in hastening decomposition, and thus preparing 
a field for invasion. Rocks and boulders (petria, petrodia, phellia) furnish 
the best examples of this reaction; cliffs (cremnia) usually have a lichen 



258 



THE FORMATION 



covering on their faces, while the forces which produce disintegration oper- 
ate from above or below. 

317. Succession by binding aeolian soils. Dunes (thinia) are classic ex- 
amples of the reaction of pioneer vegetation upon habitats of wind-borne 
sand. The initial formations in such places consist exclusively of sand- 
binders, plants with masses of fibrous roots, and usually also with strong 
rootstalks, long, erect leaves, and a vigorous apical growth. They are al- 
most exclusively perennial grasses and sedges, possessing the unique prop- 




Fig. G6. Thicket formation (Qucrcus-Holodiscus-driodium), stage V 
of the talus succession. 



erty of pushing up rapidly through a covering of sand. They react by 
fixing the sand with their roots, thus preventing its blowing about, and also 
by catching the shifting particles among their culms and leaves, forming a 
tiny area of stabilization, in which the next generation can establish a foot- 
hold. The gradual accumulation of vegetable detritus serves also to enrich 
the soil, and makes possible the advent of species requiring better nourish- 
ment. Blowouts {anemia) are almost exact duplicates of dunes in so 
far as the steps of revegetation are concerned ; while one is a hollow, and 
the other a hill, in both the reaction operates upon a wind-swept slope. 
Sand-hills (amathia) and deserts (eremia) show similar though less marked 



SUCCESSION 259 

reactions, except where they exhibit typical inland dunes. Sand-binders, 
while usually classed as xerophytic or halophytic, are in reality dissophytes. 
Their roots grow more or less superficially in moist sand, and are morpholog- 
ically mesophytic while their leaves bear the stamp of xerophytes. The di- 
rection of movement in successions of this kind is normally from xerophytes 
to mesophytes, i. e.", it is mesotropic. In sand-hills and deserts, the succes- 
sion operates wholly within the xerophytic (dissophytic) series. Along sea- 
coasts, the mesophytic terminus is regularly forest, except where forests 
are remote, when it is grassland. 

318. Succession by reducing run=off and erosion. All bare or denuded 
habitats that have an appreciable slope are subject to erosion by surface 
water. The rapidity and degree of erosion depend upon the amount of 
rainfall, the inclination of the slope, and the structure of the surface soil. 
Regions of excessive rainfall, even where the slope is slight, show great,- 
though somewhat uniform erosion ; hill and mountain are deeply eroded 
even when the rainfall is small. Slopes consisting of compact eugeogenous 
soils, notwithstanding the marked adhesion of the particles, are much eroded 
where the rainfall is great, on account of the excessive run-off. Porous 
dysgeogenous soils, on the contrary, absorb most of the rainfall; the run-off: 
is small and erosion slight, except where the slope is great, a rare condition 
on account of the imperfect cohesion of the particles. In compact soils, the 
plants of the initial formations not merely break the impact of the rain- 
drops, but, what is much more important, they delay the downward move- 
ment of the water, and produce numberless tiny streams. The delayed 
water is largely absorbed by the soil, and the reduction of the run-off pre- 
vents the formation of rills of sufficient size to cause erosion. As in dunes, 
such plants are usually perennial grasses, though composites are frequent; 
the root system is, however, more deeply seated, and a main or tap root is 
often present. On sand and gravel slopes, the loose texture of the soil re- 
sults generally in the production of sand-binders with fibrous roots. Un- 
like dunes, such slopes exhibit a large number of mats and rosettes with 
tap-roots, which are effective in preventing the slipping or washing of the 
sand, and run little danger of being covered, as is the case with dune- 
formers. In both instances, each pioneer plant serves as a center of com- 
parative stabilization for the establishment of its own offspring, and of such 
invaders as find their way in. From the nature of these, slopes almost in- 
variably pass through grassland stages before finding their termini in thick- 
ets or forests. Bad lands (tiria) furnish the most striking examples 
of eroded habitats. The rainfall in the bad lands of Nebraska and South 
Dakota is small (300 mm.) ; yet the steepness of the slope and the compact- 



260 THE FORMATION 

ness of the soil render erosion so extreme that it is all but impossible for 
plants to obtain a foothold. Their reaction is practically negligible, and the 
vegetation passes the pioneer stages only in the relatively stable valleys. 
Mountain slopes (ancia), and ridges and hills (lophia) are readily eroded 
in new or denuded areas. This is especially true of hill and mountain re- 
gions which have been stripped of their forest or thicket cover by fires, 
lumbering, cultivation, or grazing. Where the erosion is slight, the result- 
ing succession may show initial xerophytic stages, or it may be completely 
mesostatic. Excessively eroded habitats are xerostatic, as in the case of 
bad lands, or, more frequently, they are mesotropic, passing first through 
a long series of xerophytic formations. Sandbars (cheradia, syrtidia) 
should be considered here, though they are eroded by currents and waves, 
and not by run-off. They are fixed and built up by sand-binding grasses 
and sedges, usually of a hydrophytic nature, and pass ultimately into meso- 
phytic forest. 

319. Succession by filling with silt and plant remains. All aquatic hab- 
itats into which silt, wash, or other detritus is borne by streams, currents, 
floods, waves, or tides are slowly shallowed by the action of the water plants 
present. These not only check the movement of the water, thus greatly de- 
creasing its carrying power, and causing the deposition of a part or all of its 
load, but they also retain and fix the particles deposited. In accordance 
with the rule, each plant becomes the center of a stabilizing area, which rises 
faster than the rest of the floor, producing the w^ell-known hummocks of 
lagoons and swamps. All aquatics produce this reaction. It is more pro- 
nounced in submerged and amphibious forms than in floating ones, and it 
takes place more rapidly with greatly branched or dissected plants than with 
others. In pools (tiphia) and lakes (limnia), debouching streams and sur- 
face waters deposit their loads in consequence of the check exerted by the 
still water and the marginal vegetation, and delta-like marshes are quickly 
built up by filling. Springs (crenia) likewise form marshes where they gush 
forth in sands, the removal of which is impeded by vegetation. The flood 
plains and deltas of rivers show a similar reaction. The heavily laden flood 
waters are checked by the vegetation of meadows and marshes, and deposit 
most of their load. The banks of streams (ochthid) and of ditches (taphria) 
are often built up in the same fashion by the action of the marginal vegeta- 
tion upon the current. The presence of marginal vegetation often deter- 
mines the checking or deflecting of the current in such a way as to initiate 
meanders, while natural levees owe their origin to it, in part at least. Along 
low seacoasts, waves and tides hasten the deposit of river-borne detritus, 
causing the water to spread over the lowlands and form swamps. They 



SUCCESSION 



26l 



often throw back also the sediment that has been deposited in the sea, the 
marsh vegetation acting as a filter in both cases. Successions of the kind 
indicated above are regularly mesotropic. Where the soil is sandy, and the 
filling-up process sufficiently great, or where salts or humus occur in excess, 
xerophytic formations result. In certain cases, these successions appear to 
be permanently hydrostatic, changing merely from floating or submerged to 
amphibious conditions, but this is probably due to the slowness of the reac- 
tion. As a rule, the accumulation of plant remains is relatively slight, and 




Fig. 67. Pine forest formation (Pinus-xerohylium) , stage VI of the 
talus succession. 

plays an unimportant part in the reaction. In peat bogs and other extensive 
swamps, the amount of organic matter is excessive, and plays an important 
role in the building up of the swamp bed. 



320. Succession by enriching the soil. This reaction occurs to some de- 
gree in the great majority of 'all successions. The relatively insignificant 
lichens and mosses produce this result upon the most barren rocks, while 
the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it 
in marked progression. The reaction consists chiefly in the incorporation 
of the decomposed remains of each generation and each stage in the soil. A 
very important part is played by the mechanical and chemical action of the 



262 THE FORMATION 

roots in breaking up the soil particles, and in changing them into soluble 
substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play 
a large part in increasing the nutrition-content of the soil, but the extent to 
which they are effective in succession is completely unknown. The changes 
in the color, texture, and food value of the soil in passing from the initial to 
ultimate stages of a normal succession are well known, and have led many 
to think them the efficient reactions of such successions. It seems almost 
certain, however, that this is merely a concomitant, and that, even in anom- 
alous successions where facies replace each other without obvious reasons, 
the reactions are concerned more with water-content, light, and humidity 
than with the food-content of the soil. 

321. Succession by exhausting the soil. This is a reaction not at all 
understood as yet in nature. A number of phenomena, such as the "fairy 
rings" of mushrooms and other fungi, the peripheral growth and central 
decay of lichens, Lecanora, Placodium, Parmelia, and of matforming 
grasses, such as Muhlenbergia, and the circular advance of the rootstalk 
plants, indicate that certain plants at least withdraw much of the available 
supply of some essential soil element, and are forced to move away from 
the exhausted area. It is probable that the constant shifting of the in- 
dividuals of a formation year after year, a phenomenon to be discussed under 
alternation, has some connection with this. It will be impossible to establish 
such a relation, however, until the facts are exactly determined by the 
method of quadrat statistics. So far as native formations are concerned, 
there can not be the slightest question that prairies and forests have existed 
over the same area for centuries without impoverishing the soil in the least 
degree, a conclusion which is even more certain for the open vegetation of 
deserts and plains. With culture formations, the case is quite different. 
The exhaustion of the soil by continuous or intensive cultivation is a matter 
of common experience in all lands settled for a long period. Calcium, 
phosphorus, and nitrogen compounds especially are used up by crops, and 
must be supplied artificially. The reason for this difference in reaction be- 
tween native and culture formations seems evident. In harvesting, not 
merely the grain, but the stems and leaves, and in gardening often the root 
also, are removed, so that the plant makes little or no return to the soil. In 
nature, annual plants return to the ground every year all the solid matter 
of roots, stems, leaves, and fruits, with the exception of the relatively small 
number of seeds that germinate. Perennial herbs return everything but 
the persistent underground parts. Shrubs and trees replace annually an im- 
mense amount of material used in leaves and fruits, and sooner or later, by 
the gradual decay of the individuals or by the destruction of the whole 



SUCCESSION 



263 



formation, they restore all that they have taken from the soil. This balance 
is further maintained to an important degree by the activity of the roots, 
which take from the deep-seated layers of the soil the crude materials neces- 
sary for the formation of leaves and fruits. Upon the fall and decay of these, 
their materials are incorporated with the upper layers of the formation floor, 
from which they may be absorbed by the undergrowth, or find their way 
again into the layers permeated by the tree roots. From the universal oc- 
currence of weeds in cultivated regions, the pioneers in impoverished or 
exhausted fields are uniformly ruderal plants. As is well known, the seed 




Fig. G8. Spruce forest formation (Picea-Pscudotsaga-hylium), stage 
VII, the ultimate stage of the talus succession. 

production and ecesis of these forms are such that they take possession 
quickly and completely, while their demands upon the soil are of such a 
nature that the most sterile field can rapidly be covered by a vigorous growth 
of weeds. As indicated elsewhere, ruderal formations ultimately yield to 
the native vegetation, though in regions so completely given over to culture 
that native formations are lacking or remote, it is probable that successions 
reach their final stage within the group of ruderal plants. 



322. Succession by the accumulation of humus. This is the character- 
istic reaction of peat bogs and cypress swamps (oxodia), in which the 



264 THE FORMATION 

accumulation of vegetable matter is enormous. The plant remains decom- 
pose slowly and incompletely under the water, giving rise to the various 
humic acids. These possess remarkable antiseptic qualities, and have an 
injurious effect upon protoplasm. They affect the absorption of water by 
the root hairs> though this is also influenced by poor aeration. The same 
acids are found in practically all inland marshes and swamps, but the 
quantity of decomposing vegetation in many is not great enough to produce 
an efficient reaction. Formations of this type usually start as freshwater 
swamps. The succession is apparently hydrostatic, but no thorough study 
of its stages has as yet been made. 

323. Succession by modifying atmospheric factors. All layered forma- 
tions, forests, thickets, many meadows and wastes, etc., show reactions of 
this nature, and are in fact largely or exclusively determined by them. The 
reaction is a complex one, though it is clear that light is the most efficient 
of the modified factors, and that humidity, temperature, and wind, while 
strongly affected, play subordinate parts. In normal successions, the effect 
of shade, i. e., diffuse light, enters with the appearance of bushes or shrubs, 
and becomes more and more pronounced in the ultimate forest stages. The 
reaction is exerted chiefly by the facies, but the effect of this is to cause 
increasing diffuseness in each successively lower layer, in direct ratio with 
the increased branching and leaf expansion of the plants in the layer just 
above. In the ultimate stage of many forests, especially where the facies are 
reduced to one, the reaction of the primary layer is so intense as to pre- 
clude all undergrowth. Anomalous successions often owe their origin to 
the fact that certain trees react in such a way as to cause conditions in 
which they produce seedlings with increasing difficulty, and thus offer a 
field favorable to the ecesis of those species capable of enduring the dense 
shade. Successions of this kind are almost invariably mesostatic, as it is 
altogether exceptional that layered formations are either xerophytic or 
hydrophytic. 

LAWS OF SUCCESSION 

324. The investigation of succession has so far been neither sufficiently 
thorough nor systematic to permit the postulation of definite laws. Enough 
has been done, however, to warrant the formulation of a number of rules, 
which apply to the successions studied, and afford a convenient method for 
the critical investigation of all successions upon the basis of initial causes,, 
and reactions. Warming has already brought together a few such rules,, 
and an attempt is here made to reduce the phenomena of succession, includ- 
ing its causes and effects, to a tentative system. At present it is difficult to 



SUCCESSION 265 

make a thoroughly satisfactory classification of such rules, and they are 
here arranged in general conformity with the procedure in succession. 

I. Causation. The initial cause of a succession is the formation or ap- 
pearance of a new habitat, or the efficient change of an existing one. 
II. Reaction. Each stage reacts upon the habitat in such a way as to pro- 
duce physical conditions more or less unfavorable to its permanence, 
but advantageous to the invaders of the next stage. 

III. Proximity and mobility. 

(1) The pioneers of a succession are those species nearest at hand 

that are the most mobile. 

(2) The number of migrants from any formation into a habitat varies 

inversely as the square of the distance. 

(3) The pioneer species are regularly derived from different forma- 

tions, as the latter nearly always contain permobile species 
capable of effective ecesis. 

(4) The plants of the initial stages are normally algae and fungi, 

with minute spores, composites, and grasses, which possess 
permobile fruits, or ruderal plants, on account of their great 
seed production. 

IV. Ecesis. 

( 1 ) All the migrants into a new, denuded, or greatly modified habitat 

are sorted by ecesis into three groups: (1) those that are 
unable to germinate or grow, and soon die; (2) those that 
grow normally under the conditions present; (3) those that 
pass through one or more of the earlier stages in a dormant 
state to appear at a later stage of the succession. 

(2) Wherever ruderal vegetation is present, it contributes a large 

number of the pioneer species of each succession, on account 
of the thorough ecesis. In other regions this part is played 
by subruderal native species. 

(3) Annuals and biennials are characteristic of the early stages of 

secondary successions, on account of their great seed produc- 
tion and ready ecesis. 

(4) In layered formations, heliophytes appear before sciophytes; 

they ultimately yield to the latter, except where they are able 
to maintain a position in the primary layer. 

(5) Excessive seed production and slight mobility lead to the im- 

perfect ecesis of individuals in dense stands, and in conse- 
quence usually produce great instability. 



266 THE FORMATION 

(6) Each pioneer produces about itself a tiny area of ecesis and 

stabilization for its own offspring, for the disseminules of its 
fellows, or of invaders. 

(7) Species propagating by offshoots, or producing relatively im- 

mobile disseminules in small number, usually show effective 
ecesis, as the offspring appear within the area of the reaction 
of the parent forms. 
V. Stabilization. 

(1) Stabilization is the universal tendency of vegetation. 

(2) The ultimate stage of a succession is determined by the domi- 

nant vegetation of the region. Lichen formations are often 
ultimate in polar and niveal zones ; grassland is the final vege- 
tation for plains and alpine stretches, and for much prairie, 
while forest is the last stage for mesophytic midlands and 
lowlands, as well as for subalpine regions. 

(3) Grassland or forest is the usual terminus of a succession; they 

predominate in lands physiographically mature. 

(4) The limit of a succession is determined in large part by the pro- 

gressive increase in occupation, which makes the entrance of 
invaders more and more difficult. 

(5) Stabilization proceeds radiately from the pioneer plants or 

masses. The movement of offshoots is away from the parent 
mass, and the chances of ecesis are greatest near its edges, in 
a narrow area in which the reaction is still felt, and the 
occupation is not exclusive. 
VI. General laws. 

(1) The stages, or formations, of a succession are distinguished as 

initial (prodophytia), intermediate (ptenophytia), and ulti- 
mate (aiphytia). 

(2) Initial formations are open, ultimate formations are closed. 

(3) The number of species is small in the initial stages; it attains a 

maximum in intermediate stages ; and again decreases in the 
ultimate formation, on account of the dominance of a few 
species. 

(4) The normal sequence of vegetation forms in succession is: (1) 

algae, fungi, mosses; (2) annuals and biennials; (3) peren- 
nial herbs; (4) bushes and shrubs; (5) trees. 

(5) The number of species and of individuals in each stage increases 

constantly up to a maximum, after which it gradually de- 
creases before the forms of the next stage. The interval 
between two maxima is occupied by a mixed formation. 



SUCCESSION 267 

(6) A secondary succession does not begin with the initial stage of 

the primary one which it replaces, but usually at a much later 
stage. 

(7) At present, successions are generally mesotropic, grassland and 

forest being the ultimate stages, though many are xerostatic 
or hydrostatic If erosion continue until the sea level is 
reached, the ultimate vegetation of the globe will be hydro- 
phytic. Should the heat of the sun decrease greatly before 
this time, the last vegetation will be xerophytic, i. e., 
crymophytic. 

(8) The operation of succession was essentially the same during the 

geological past as it is to-day. From the nature of their 
vegetation forms, the record deals largely with the ultimate 
stages of such successions. 

CLASSIFICATION AND NOMENCLATURE 

325. Basis. New or denuded habitats arise the world over by the opera- 
tion of the same or similar causes, and they are revegetated in consequence 
of the same reactions. Similar habitats produce similar successions. The 
vegetation forms and their sequence are usually identical, and the genera 
are frequently the same, or corresponding in regions not entirely unrelated. 
The species are derived from the adjacent vegetation, and, except in alpine 
and coast regions, are normally different. The primary groups of succes- 
sions are determined by essential identity of habitat or cause, e. g., aeolian 
successions, erosion successions, burn successions, etc. When they have 
been more generally investigated, it will be possible to distinguish subordi- 
nate groups of successions, in which the degree of relationship is indicated 
by the similarity of vegetation forms, the number of common genera, etc. 
For example, burn successions in the Ural and in the Rocky mountains 
show almost complete similarity in the matter of vegetation forms and their 
sequence, and have the majority of their genera in common. A natural 
classification of successions will divide them first of all into normal and 
anomalous. The former fall into two classes, primary and secondary, and 
these are subdivided into a number of groups, based upon the cause which 
initiates the succession. 

326. Nomenclature. The need of short distinctive names of interna- 
tional value for plant formations is obvious; it has become imperative that 
successions also should be distinguished critically and designated clearly. 
From the very nature of the case, it is impossible to designate each forma- 
tion or succession by a single Greek or Latin term, as habitats of the same 



268 



THE FORMATION 



character will show in different parts of the world a vegetation taxonomically 
very different. It may some day be possible to use a binomial or trinomial 
for this purpose, somewhat after the fashion of taxonomy, in which the 
habitat name will represent the generic idea as applied to formations, and 
a term drawn from the floristic impress the specific idea. Such an attempt 
would be futile or valueless at the present time ; it could not possibly meet 
with success until there is more uniformity in the concept of the formation, 
-and until there has been much accurate and thorough investigation of actual 
formations, a task as yet barely begun. At present, it seems most feasible 
as well as scientific to designate all formations occupying similar habitats 




Fig. 69. Aspen forest formation (Populus-hylium), the typical stage 
of burn successions in the Rocky mountains; it is sometimes an anom- 
alous stage in primary successions, interpolated in place of the thicket 
formation. 

by a name drawn from the character of the latter, such as a meadow forma- 
tion, poium, a forest formation, hylium, a desert formation, eremium, etc. 
A particular formation is best designated by using the generic name of one 
or two of its most important species in conjunction with its habitat term, 
as Spartina-Elymus-poium, Picea-Piniis-hylinm, Cereus-Yucca-eremium, etc. 
Apparently a somewhat similar nomenclature is adapted to successions. 
The cause which produces a new habitat may well furnish the basis for 
the name of the general groups of successions, as pyrium (literally, a place 



SUCCESSION 269 

or a habitat burned over), a burn succession, tribium, an erosion succession, 
etc. A burn succession consists of a sequence of certain formations in one 
part of the world, and of a series of quite different ones, floristically, in 
another. A particular burn succession should be designated by using the 
names of a characteristic facies of the initial and ultimate stages in con- 
nection with the general term, e. g., Bryum-Picea-pyrium, etc. A trinomial 
constructed in this way represents the desirable mean between definition and 
brevity. Greater definiteness is possible only at the expense of brevity, while 
to shorten the name would entirely destroy its precision. The following 
classification of successions is proposed, based upon the plan outlined above. 
The termination -iutn (e?ov) has been used throughout in the construction 
of names for successions, largely for reasons of euphony. If it should be- 
come desirable to distinguish the names of formations and successions by 
the termination, the locative suffix -on ( -wv ) should be used for the latter. 
The terms given below would then be hypson, rhyson, hedon, sphyron, 
prochoson, pnoon, pagon, tribon, clyson, rcpon, olisthon, xerasion, theron, 
broton, pyron, ecballon, camnon, ocheton, ardon. 

I. Normal successions: cyriodochae (Kvpios, regular, 80^, r), succession) 
a Primary successions: protodochae (ttp&tos, first, primary) 

1. By elevation: hypsium (inf/os, to, height, elevation, -eiov, place) 

2. By volcanic action: rhysium (/Wis r), flowing, especially of fire) 

3. In residuary soils: hedium (e8os, to, a sitting base) 

4. In colluvial soils: sphyrium (o-^vpov, to, ankle, talus) 

5. In alluvial soils: prochosium (irp6yk>(TL<;, rj, a deposition of mud) 

6. In aeolian soils: pnoium {irvorj, r), blowing, blast) 

7. In glacial soils: pagium (7rayo?, 6, that which becomes solid, i.e., a 

glacier) 
b. Secondary successions: hepodochae (1™, to follow) 

8. In eroded soils: tribium (rpt^w, wear or rub away) 

9. In flooded soils: clysium (kXvotls, 6, a drenching, flooding) 

10. By subsidence: repium (peVw, incline downwards, sink) 

11. In landslips: olisthium (oAio-0os, 6, slip) 

12. In drained and dried out soils: xerasium (irjpao-ta, r), drought) 

13. By animal agencies: therium {Orjp, 6, wild animal) 

14. By human agency: brotium (yS/oords, 6, a mortal) 

a. Burns: pyrium (nvp, to, fire) 

b. Lumbering: ecballium (€K(3d\X<a, cut down forests) 

c. Cultivation: camnium (kol/j.vo), cultivate) 

d. Drainage: ochetium (oxeros, 6, drain) 
€. Irrigation: ardium (apSco, irrigate) 



2 7° THE FORMATION 

II. Anomalous successions: xenodochae (£*vo>, strange, unusual) 

327. Illustrations. The following series will illustrate the application of 
this system of nomenclature to particular successions, and their stages, or 
formations. 

Thlaspi-Picea-sphyrium : pennycress-spruce talus succession 

Thlaspi-Eriogonum-chaiicium : pennycress-eriogonum gravel slide forma- 
tion. 
Elymus-Gilia-chalicium : wildrye-gilia half gravel slide formation 
Quercus-Hoiodiscus-driodium : oak-fringewood dry thicket formation 
Pinus-xerohylium : pine dry forest formation 
Picea-Pseudotsuga-hylium : spruce-balsam forest formation 

Bryum-Picea-pyrium : moss-spruce burn succession 
Bryum-telmatium : moss meadow formation 
Aster-Chamaenerium-poium : aster- fireweed meadow formation 
Deschampsia-Carex-poium : hairgrass-sedge meadow formation 
Salix-Betula-helodrium : willow-birch meadow thicket formation 
Populus-hylium : aspen forest formation 
Picea-hylium : spruce forest formation 

Lecanora-Carex-hedium : lichen-carex residuary succession 
Lecanora-Gyrophora-petrium : crustose lichen rock formation 
Parmelia-Cetraria-chalicium : foliose lichen gravel slide formation 
Paronychia-Silene-chalicium : nailwort-campion gravel slide formation 
Carex-Campanula-coryphium : sedge-bluebell alpine meadow formation 

Eragrostis-Helianthus-xerasium : eragrostis-sunflower drainage succession 
Eragrostis-Polygonum-telmatium : eragrostis-heartsease wet meadow for- 
mation 
Helianthus-Ambrosia-chledimn : sunflower-ragweed waste formation 

INVESTIGATION OF SUCCESS/ON 

328. General rules. The study of succession must proceed along two 
fundamental lines of inquiry : it is necessary to investigate quantitatively 
the physical factors of the initial stages and the reactions produced by the 
subsequent stages. This should be done by automatic instruments for 
humidity, light, temperature, and wind, in order that a continuous record 
may be obtained. Water-content is taken daily or even less frequently, while 
soil properties, and physiographic factors, altitude, slope, surface, and ex- 
posure are determined once for all. It is equally needful to determine the 
development and structure of each stage with particular reference to the 
adjacent formations, to the stage that has just preceded, and the one that is 



SUCCESSION 



27I 



to follow. For this, the use of the permanent quadrat is imperative, as the 
sequence and structure of the stages can be understood only by a minute 
study of the shifting and rearrangement of the individuals. Permanent mi- 
gration circles are indispensable for tracing movement away from the pioneer 
areas by which each stage reaches its maximum. Denuded quadrats are a 
material aid in that they furnish important evidence with respect to migration 
and ecesis, By means of them, it is possible to determine the probable devel- 




Fig. 70. Alternating gravel slides on Mounts Cameron and Pals- 
grove, from the comparison of which the initial development of the 
talus succession has been reconstructed. 

opment of stages which reach back a decade or more into the past. In the ex- 
amination of successions, since cause and effect are so intimately connected in 
each reaction, it is especially important that general and superficial observa- 
tions upon structure and sequence be replaced by precise records, and that 
vague conjectures as to causes and reactions be supplanted by the accurate 
determination of the physical factors which underlie them;. 



329. Method of alternating stages. The period of time through 
which a primary succession operates is usually too great to make a complete 
study possible within a single lifetime. Secondary successions run their 
course much more quickly, and a decade will sometimes suffice for stabiliza- 



2J2 THE FORMATION 

tion, though even here the period is normally longer. The longest and most 
complex succession, however, may be accurately studied in a region, where 
several examples of the same succession occur in different stages of de- 
velopment. In the same region, the physical factors of one example of a 
particular succession are essentially identical with those of another example 
in the same stage. If one is in an initial stage, and the other in an interme- 
diate condition, the development of the former makes it possible to re- 
establish more or less completely the life history of the latter. The same 
connection may be made between intermediate and ultimate stages, and it 
is thus possible to determine with considerable accuracy and within a few 
years the sequence of stages in a succession that requires a century or more 
for its complete development. In the Rocky mountains, gravel slides (talus 
slopes) are remarkably frequent. They occur in all stages of development, 
and the alternating slides of different ages furnish an almost perfect record 
of this succession. This method lacks the absolute finality which can be 
obtained by following a succession in one spot from its inception to final 
stabilization, but it is alone feasible for long successions, i. e., those extending 
over a score or more of years. When it comes to be universally recognized 
as a plain duty for each investigator to leave an exact and complete record 
in quadrat maps and quadrat photographs of the stages studied by him, it 
will be a simple task for the botanists of one generation to finish the investi- 
gations of succession begun by their predecessors. 

330. The relict method of studying succession is next in importance 
to the method of alternating areas. The two in fact are supplementary, 
and should be used together whenever relicts are present. This method 
is based upon the law of successive maxima, viz., the number of species and 
of individuals in each stage constantly increases up to a certain maximum, 
after which it gradually decreases before the forms of the next stage. In 
accordance with this, secondary species usually disappear first, principal 
species next, and facies last of all. There are notable exceptions to this, how- 
ever, and the safest plan is to use the relict method only when principal 
species or facies are left as evidence. An additional reason for this is that 
secondary species are more likely to be common to two or more formations. 
In the majority of cases, the relict is not modified, and is readily recognized 
as belonging properly to a previous stage. This is true of herbs in all the 
stages of grassland, and in the initial ones of forest succession. The herbs 
and shrubs of earlier stages, which persist in the final forest stages, are 
necessarily modified, often in such a degree as to become distinct ecads, or 
species. The facies of the stages which precede the ultimate forest are rarely 
modified. The application of the relict method, together with the modifica- 



SUCCESSION 



273 



tion just described, is nicely illustrated by the balsam-spruce formation at 
Minnehaha. Of the initial gravel slide stage, the relicts are Vagnera stellata 
and Galium boreale, the one modified into Vagnera leptopetala, and the 
other into G. boreale hylocolum. The thicket stage is represented by Holo- 
discus dumosa, greatly changed in form and branching, and in the shape 
and structure of the leaf. The most striking relict of the aspen formation 
is the facies itself, Populus tremuloides. The tall slender trunks of dead 
aspens are found in practically every balsam-spruce forest. In many places, 
living trees are still found, with small, straggling crowns, which are vainly 
trying to outgrow the surrounding conifers. Of the aspen undergrowth, 




Fig. 71. Relict spruces and aspens, showing the character of the suc- 
cession immediately preceding the burn succession now developing. 

Rosa sayii, Helianthella parryi, Frasera speciosa, Zygadenns elegans, Cas- 
tilleia confusa, Gentiana acuta, and Solidago orophila remain more or less 
modified by the diffuse light. It is still a question whether the aspen stage 
passes directly into the balsam-spruce forest, or whether a pine forest inter- 
venes. The presence of both Pinus ponderosa and P. Hexilis, which are 
scattered more or less uniformly through the formation, furnishes strong 
evidence for the latter view. 

The lifetime of forest and thicket stages of successions is ascertained by 
counting the annual rings of the stumps of facies. This is a perfectly feasible 



274 THE FORMATION 

method for many woodland formations where stumps already abound or 
where a fire has occurred, and it is but rarely necessary to cut down trees 
for this purpose. When trees or shrubs are present as relicts, the same 
method is used to determine the length of time taken by the development of 
the corresponding stages. 

THE STRUCTURE OF THE FORMATION 

331. Since all the structures exhibited by formations, such as zones, lay- 
ers, consocies, etc., are to be referred to zonation or alternation, these princi- 
ples are first considered in detail. This, then, constitutes the basis for a con- 
sideration of the structure of a normal formation, with special reference to 
the different parts that compose it. The investigation of formational struc- 
ture, since the latter is the result of aggregation, invasion, and succession, is 
accomplished by instruments, quadrats, etc., in the manner already indicated 
under development, and no further discussion of it is necessary here. 

ZONATION 

332. Concept. The recognition of vegetation zones dates from Tourne- 
fort 1 , who found that, while the plants of Armenia occupied the foot of 
Mount Ararat, the vegetation of the slopes above contained many species of 
southern Europe. Still higher appeared a flora similar to that of Sweden, 
and on the summit grew arctic plants, such as those of Lapland. 

As the historical summary shows, the concept of zonation is the oldest in 
phytogeography. Notwithstanding this, it has never been clearly defined, 
nor has there been any detailed investigation of the phenomenon itself, or 
of the causes which produce it. Zones are so common, and often so clearly 
marked, that they invite study, but no serious attempt has heretofore been 
made to analyze zonation, or to formulate a definite method of investigating 
it. Zonation is the practically universal response of plants to the quantita- 
tive distribution of physical factors in nature. In almost all habitats, one or 
more of the physical factors present decreases gradually in passing away 
from the point of greatest intensity. The result is that the plants of the 
habitat arrange themselves in belts about this point, their position being de- 
termined by their relation to the factor concerned. Close investigation will 
show that there is hardly a formation that is entirely without zonation, 
though in many cases the zones are incomplete or obscure for various reasons. 
Zonation is as characteristic of vegetation as a whole as it is of its unit, 
the formation, a fact long ago recognized in temperature zones. A conti- 

1 Relation d'un Voyage du Levant. 1717. 



ZO NATION 275 

nental climate, however, often results in the interruption of these, with the 
consequence that these belts of vegetation are not always continuous. 

CAUSES OF ZONATION 

333. Growth. The causes that produce zones are either biological or 
physical : the first have to do with some characteristic of the plant, the 
second with the physical features of the habitat. Biological causes arise from 
the method of growth, from the manner of dissemination, or from the re- 
action of the species upon the habitat. The formation of circles as a result 
of radial growth is a well-known occurrence with certain plants, but it is 
much more common than is supposed. In the case of agarics, this phenom- 
enon has long been known under the name of "fairy-rings." It is found in 
a large number of moulds, and is characteristic of early stages of the mycel- 
ium of the powdery mildews. It occurs in nearly all maculicole fungi, and 
is exhibited by certain xylogenous fungi, such as Hysterographium. Among 
the foliose lichens, it is a common occurrence with the rock forms of 
Parmelia, Placodium, Physcia, and Lecanora, and with the earth forms of 
Parmelia and Peltigera. The thalloid liverworts show a similar radial 
growth. The flowering plants, and many mosses also, furnish good examples 
of this sort of growth in those species which simulate the form of the my- 
celium or thallus. These are the species that form mats, turfs, or carpets. 
Alpine mat formers, such as Silcne acaulis, Paronychia pulvinata, Arenaria 
sajanesis, etc., are typical examples. Xerophytic, turf-forming species of 
Muhlenbergia, Sporobolus, Boutelona, Festuca, Poa, and other grasses form 
striking ring-like mats, while creeping species of Euphorbia, Portulaca, 
Amarantus, etc., produce circular areas. Rosettes, bunch-grasses, and many 
ordinary rootstalk plants spread rapidly by runners and rhizomes. The 
direction of growth is often indeterminate in these also, and is in consequence 
more or less bilateral or unilateral. Growth results in zonation only when 
the older central portions of the individual or mass die away, leaving an 
ever-widening belt of younger plants or parts. This phenomenon is doubt- 
less due in part to the greater age of the central portion, but seems to arise 
chiefly from the demands made by the young and actively growing parts 
upon the water of the soil. There may possibly be an exhaustion of nutritive 
content, as in the case of the fungi, but this seems improbable for the reason 
that young plants of the same and other species thrive in these areas. It 
must not be inferred that these miniature growth zones increase in size until 
they pass into zones of formations. Growth contributes its share to the 
production of these, but there is no genetic connection between a tiny plant 
zone and a zone of vegetation. 



2j6 THE FORMATION 

Radial and bilateral growth play an important part in formational zones 
in so far as they are related to migration. The growth of the runner or 
rhizome itself is a very effective means of dissemination, while the seeding 
of the plants thus carried away from the central mass is most effective at the 
edge of the newly occupied area. This holds with equal force for plants 
with a mycelium or a thallus. The circular area becomes larger } r ear by year. 
Sooner or later, the younger, more vigorous, and more completely occupied 
circumference passes into a more, or less complete zone. • This will result 
from the reaction of the central individuals upon the habitat, so that they 
are readily displaced by invaders, or from their increasing senility and 
dying out, or from the invasion of forms which seed more abundantly and 
successfully. This result will only be the more marked if the radiating 
migrants reach a belt of ground especially favorable to their ecesis. In this 
connection it must be carefully noted that vegetation pressure, before which 
weaker plants are generally supposed to flee, or by which they are thought 
to be forced out into less desirable situations, is little more than a fanciful 
term for radial growth and migration. It has been shown under invasion 
that disseminules move into vegetation masses, as well as away from them, 
the outward movement alone being conspicuous, because it is only at the 
margin and beyond that they find the necessary water and light for growth. 

334. Reactions. Certain reactions of plants upon habitats produce zon- 
ation. The zones of fungi are doubtless caused by the exhaustion of the 
organic matter present, while in lichens and mosses the decrease in nutritive 
content has something to do with the disappearance of the central mass. In 
the mats of flowering plants, the connection is much less certain. The re- 
action of a forest or thicket, or even of a tall herbaceous layer, is an ex- 
tremely important factor in the production of zonation. The factor chiefly 
concerned here is light. Its intensity is greatest at the edge of the formation 
and just below the primary layer; the light becomes increasingly diffuse 
toward the center of the forest, and toward the ground. In response to this, 
both lateral and vertical zones appear. The former are more or less incom- 
plete, and are only in part due to differences in illumination. The vertical 
zones or layers are characteristic of forest and thickets, and are caused 
directly by differences in light intensity. 

335. Physical factors. The physical causes of zonation are by far the 

most important. They arise from differences in temperature, water, and 
light. In the large, temperature differences are the most important, pro- 
ducing the great zones of vegetation. In a particular region or habitat, 
variations of water-content and humidity are controlling, while light, as 



ZO NATION 



277 



shown above, is important in the reactions of forest and thicket. Physical 
factors produce zonation in a habitat or a series of habitats, when there is 
either a gradual and cumulative, or an abrupt change in their intensity. 
Gradual, slight changes are typical of single habitats ; abrupt, marked 
chang'es of a series of habitats. This modification of a decisive factor tends 
to operate in all directions from the place of greatest intensity, producing 
a characteristic symmetry of the habitat with reference to the factor con- 
cerned. If the area of greatest amount is linear, the shading-out will take 





-£J 



Fig. 72. Zones of Cyperus erythrorrhisus produced by the recession 
of the shore-line. 



place in two directions, and the symmetry will be bilateral, a condition well 
illustrated by rivers. On the other hand, a central intense area will shade 
out in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. 
The essential connection between these is evident where a stream broadens 
into a lake, or the latter is the source of a stream, where a mountain ridge 
breaks up into isolated peaks, or where a peninsula or landspit is cut into 
islands. The line that connects the points of accumulated or abrupt change 
in the symmetry is a stress line or c cot one. Ecotones are well-marked be- 
tween formations, particularly where the medium changes; they are less 
distinct within formations. It is obvious that an ecotone separates two 



278 



THE FORMATION 



different series of zones in the one case, and merely two distinct zones in the 
other. 

336. Physiographic symmetry. The physical symmetry of a habitat 
depends upon the distribution of water in it, and this is profoundly affected 
by the soil and the physiography. The influence of precipitation is slight or 
lacking, as it is nearly uniform throughout the habitat; the effects of wind 
and humidity are more localized. Differences of soil rarely obtain within 
a single habitat, though often occurring in a zoned series. The strikingly 




Fig. 73. Regional zones on a spur of Pike's Peak (3,800 m.) ; the 
forest consists of Picea engelmannii and Pinus aristata, the forewold is 
Salix pseudolapponum, and the grassland, alpine meadow (Carex-Cam- 
panula-coryphium). 

zonal structure or arrangement of habitats is nearly always due to differ- 
ences in water-content produced by physiographic factors, slope, exposure, 
surface, and altitude. The effect of these upon water-content and humidity 
is obvious. Wherever appreciable physiographic differences occur, there 
will be central areas of excess and deficiency in water-content, between 
which there is a symmetrical modification of this factor. Peaks are typical 
examples of areas of deficiency, lakes and oceans of areas of excess. When 
these areas are extreme and close to each other, the resulting zonation will be 
marked; when they are moderate, particularly if they are widely separated, 



ZONATION 279 

the zones produced are obscure. Asymmetry of a habitat or a region prac- 
tically does not exist. Central areas of excess and deficiency may be very 
large and in consequence fail to seem symmetrical, or the space between 
them so great that the symmetry is not conspicuous, but they are everywhere 
present, acting as foci for the intervening areas. 

The response of vegetation to habitat is so intimate that physiographic 
symmetry everywhere produces vegetational symmetry, which finds its ready 
expression in plant zones. The reaction of vegetation upon habitat causes 
biological symmetry, typical of growth zones and light zones. From these 
facts it is clear that zonation will be regularly characteristic of the vegetative 
covering. The zonal arrangement of formations is usually very evident; 
the zones of a formation are often obscured, or, where the latter occupies 
a uniform central area of excess or deficiency, they are rudimentary or lack- 
ing, as in shallow ponds. Zones are frequently imperfect, though rarely 
entirely absent in new soils, such as talus. They are rendered obscure in 
several ways. In the initial stages of a succession, as well as in the transi- 
tions between the various stages, the plant population is so scattered, so 
transient, or so dense as to respond not at all to a degree of symmetry 
which produces marked zonation in later formations. The alternation of 
conspicuous species not only causes great interruption of zones, but often 
also completely conceals the zonation of other species, such as the grasses, 
which, though of more importance in the formation, have a lower habit of 
growth. Furthermore, the ecotones of one factor may run at right angles to 
those of another, and the resulting series of zones mutually obscure each 
other. Finally, such a physiographic feature as a hill may have its sym- 
metry interrupted by ridges or ravines, which deflect the zones downward 
or upward, or cause them to disappear altogether, while the shallows or 
depths of a pond or lake may have the same effect. An entire absence of 
zones, i. e., azonation, is exceptional in vegetation. Almost all cases that 
seem to exhibit it may be shown by careful examination to arise in one of the 
several ways indicated above. 

KINDS OF ZONATION 

337. Two kinds of zonation are distinguished with reference to the 
direction in which the controlling factor changes. When this is horizontal, 
as with water-content and temperature, zonation will be lateral; when it is 
vertical, as in the case of light, the zonation is vertical. There exists an 
intimate connection between the two in forests, where the secondary layer 
of small trees and shrubs is continuous with a belt of trees and shrubs around 
the central nucleus, and the lower layers of bushes and herbaceous plants 
with similar zones still further out. This connection doubtless arises from 



280 THE FORMATION 

the fact that conditions are unfavorable to the fades, outside of the nucleus 
as well as beneath it. Floristically, each layer and its corresponding zone 
are distinct, as the one consists of shade, the other of sun species. Lateral 
zonation is radial when the habitat or physiographic feature is more or less 
circular in form, and it is bilateral, when the latter is elongated or linear. 
Vertical zonation is unilateral. 

338. Radial zonation is regularly characteristic of elevations and de- 
pressions. From the form of the earth, it reaches its larger expression in 
the girdles of vegetation corresponding to the zones of temperature. The 
zones of mountain peaks are likewise due largely to temperature, though 
humidity is a very important factor also. Mountain zones are normally 
quite perfect. The zonation of islands, hills, etc., is due to water-content. 
In the former, the zones are usually quite regular and complete ; in the 
latter, they are often incomplete or obscured. Prairies and steppes are not 
zoned as units, but are complexes of more or less zonal hills and ridges. 
Ponds, lakes, and seas regularly exhibit complete zones, except in those 
shallow ponds where the depth is so slight that what is ordinarily a marginal 
zone is able to extend over the entire bottom. The line between an elevation 
and a depression, i. e., the edge of the water level, is the most sharply 
defined of all ecotones. It separates two series of zones, each of which con- 
stitutes a formation. One of these is regularly hydrophytic, the other is 
usually mesophytic. The line between the two can rarely be drawn at the 
water's edge, as this is not a constant, owing to waves, tides, or periodical 
rise and fall. There is in consequence a more or less variable transition 
zone of amphibious plants, which are, however, to be referred to the hydro- 
phytic formation. Nearly all forest formations serve as a center about which 
are arranged several somewhat complete zones. As a rule, these merge 
into a single heterogeneous zone of thickets. 

339. Bilateral zonation differs from radial only in as much as it deals 
with linear elevations and depressions instead of circular ones. With this 
difference, the zones of ranges and ridges correspond exactly to those of 
peaks and hills, while the same relation is evident between the zones of 
streams, and of lakes and ponds. The ecotones are identical except as to 
form ; they are linear in the one and circular in the other. Incompleteness is 
more frequently found in bilateral zonation, though this is a question of 
distance or extent, rather than one of symmetry. 

340.- Vertical zonation is peculiar in that there is no primary ecotone 
present, on either side of which zones arrange themselves with reference to 



ZONATION 28l 

the factor concerned. This arises from the fact that the controlling factor 
is light, which impinges upon the habitat in such manner as to shade out in 
but one direction, i. e., downward. Vertical zones appear in bodies of 
water, on account of the absorption of light by the water. In a general way, 
it is possible to distinguish bottom, plancton, and surface zones, consisting 
almost wholly of algae. There is little question that minor zones exist, es- 
pecially in lakes and seas, but these await further investigation. The most 
characteristic vertical zones occur in forests, where the primary layer of trees 
acts as a screen. The density of this screen determines the number of zones 
found beneath it. In extreme cases the foliage is so dense that the light 
beneath is insufficient even for mosses and lichens. As a rule, however, 
there will be one. or more zones present. In an ordinary deciduous forest, 
the layers below the facies are five or six in number : ( 1 ) a secondary layer 
of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper 
herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a lower 
herbaceous layer, (6) a ground layer of mosses, lichens, other fungi, and 
algae. The upper layers are often discontinuous, the lower ones are more 
and more continuous. As a forest becomes denser, its layers disappear from 
the upper downward, the ground layer always being the last to disappear 
because of its ability to grow in very diffuse light. A vertically zoned for- 
mation shows a complex series of reactions. The primary layer determines 
the amount of heat, light, water, wind, etc., for the subordinate layers in 
general. Each of these layers then further determines the amount for those 
below it, the ground layer being subject in some degree to the control of 
every layer above it. This accounts probably for the defmiteness and per- 
manence of this layer. The degree to which the lower layers influence the 
upper by reacting upon the habitat is not known. It is evident that this in- 
fluence must be considerable by virtue of their control of the water supply in 
the upper soil strata, by virtue of their transpiration, their decomposition, etc. 
The ecotone between two formations is never a sharp line, but it is an 
area of varying width. The edge of this area which is contiguous to one 
formation marks the limit for species of the other. Both formations dis- 
appear in this transition zone, but in opposite directions. The overlapping 
which produces such zones arises from the fact that the physical factors 
tend to approach each other at the line of contact between formations, and 
that many species are more or less adjustable to conditions not too dissimilar. 

341. Vegetation zones. As a fundamental expression of progressive 
change in the amount of heat and water, zonation is the most important 
feature of vegetation. It constitutes the sole basis for the division of con- 
tinental as well as insular vegetation. The continent of North America 



282 THE FORMATION 

furnishes striking proof of the truth of this. Conforming to the gradual 
decrease of temperature and water-content northward, three primary belts 
of vegetation stretch across the continent from east to west. These are 
forest, grassland, and polar desert. The first is further divided into the 
secondary zones of broad-leaved evergreen, deciduous, and needle-leaved 
forests. At right angles to this temperature-water symmetry lies a symmetry 
due to water alone, in accordance with which forest belts touch the oceans, 
but give way in the interior to grasslands, and these to deserts. It is at 
once evident that the mutual interruption of these two series of zones has 
produced the primary features of North America vegetation, i. e., tropical 
forests where heat and water are excessive, deserts where either is unusually 
deficient, grassland when one is low, the other moderate, and deciduous and 
coniferous forests, where the water-content is as least moderate and the 
temperature not too low. Such a simple yet fundamental division has been 
modified, however, by the disturbing effect which three continental moun- 
tain systems have had upon humidity and upon temperature symmetry. The 
two are intimately interwoven. The lowering of temperature due to altitude 
produces the precipitation of the wind-borne moisture upon those slopes 
which look toward the quarter from which the prevailing winds blow. A 
mountain range thus makes an abrupt change in the symmetry, and renders 
impossible the gradual change from forest to grassland and desert. The 
Appalachian system is not sufficiently high to produce a pronounced effect, 
and forests extend far beyond it into the interior before passing into prairies 
and plains. On the other hand, the influence of the Rocky mountains and the 
Sierra Nevada is very marked. The latter rise to a great height relatively 
near the coast, and condense upon their western slopes nearly all of the 
moisture brought from the Pacific. The Rocky mountains have the same 
effect upon the much drier winds that blow from the east, and the two sys- 
tems in consequence enclose a parched desert. This series of major zones 
thus becomes, starting at the east, forest, grassland, desert, and forest, in- 
stead of the more symmetrical series, forest, grassland, desert, grassland, for- 
est, which would prevail were it not for these barriers. This actual series of 
major zones undergoes further interruption by the action of these mountain 
systems in deflecting northern isotherms far to the south. This action is 
greatest in the high ranges, the Rocky mountains and the Sierras, and least 
in the lower Appalachians. Its result is to carry the polar deserts of the 
north far southward along the crests of the mountains, and to extend the 
boreal coniferous forests much further south along their slopes. In the 
Appalachians, this means no more than the extension of a long tongue of 
conifers into the mass of deciduous forests, and the occasional appearance 
of an isolated peak. In the western ranges, it produces two symmetrical 



ALTERNATION 283 

series of minor mountain zones, forest, alpine grassland or desert, and 
forest, to say nothing of the foot-hill and timber-line zones of thicket. 

There seems to be no good reason for distinguishing the zones of moun- 
tains as regions. The term itself is inapplicable, as it has no reference to 
zonation, and is used much more frequently as a term of general application. 
Its use tends to obscure also the essential identity of the so-called vertical 
zones of mountains with the major continental zones, an identity which can 
not be insisted upon too strongly. For the sake of clearness, it is important 
to distinguish all belts of vegetation as zones, though it is evident that these 
are not all of the same rank. The following division of the vegetation of 
North America is based upon the fundamental principles of continental sym- 
metry and the community of continental and mountain zones. 

I. Polar-niveal zone — zona polari-nivalis 

II. Arctic-alpine zone — zona arctici-alpina 
Arctic province — provincia arctica 
Alpine province — provincia alpina 

III. Boreal-subalpine zone — zona boreali-subalpina 

Alaska province — provincia alaskana 
Cordilleran province — provincia cordillerana 
Ontario province — provincia ontariensis 

IV. Temperate zone — zona temperata 

Atlantic, province — provincia atlantica 
Appalachian province — provincia appalachiana 
Nebraska province — provincia nebraskensis 
Utah province — provincia utahensis 
Coast province — provincia litoralis 
Pacific province — provincia pacifica 

V. Subtropical zone — zona subtropicalis 

Florida province — provincia floridana 
Mexican province — provincia mexicana 

VI. Tropical zone — zona tropicalis 

Antilles province — provincia antilleana 
Andean province — provincia andeana 

ALTERNATION 

342. Concept. The term alternation is used to designate that phenom- 
enon of vegetation, in which a formation recurs at different places in a 



284 THE FORMATION 

region, or a species at separate points in a formation. Although it is a funda- 
mental feature of vegetation, it has been recognized but recently. 1 

Alternation is the response of vegetation to the heterogeneity of the sur- 
face of the earth. It is in sharp contrast to zonation, inasmuch as it is directly 
caused by asymmetry in the topography. In consequence, it deals with the 
subdivisions of zones, arising from physical differences within the sym- 
metrical area. It deals with vegetation areas of every rank below that of 
major zone, with the habitat and geographical areas of species, and, in a 
certain way, with the correspondence of vicarious genera. The breaking up 
of vegetation into formations is a striking example of alternation. The same 
phenomenon occurs in every formation, producing consocies and minor 
plant groups, and everywhere giving variation to its surface and structure. 
The essential idea involved in this principle is the recurrence of like forma- 
tions, consocies, or groups, which are more or less separated by forma- 
tions, consocies, or groups differing from them. It is an exact expression 
of the primary law of association that heterogeneity of structure varies 
directly as the extent and complexity of the habitat, or the series of habitats. 
Vegetation is made up of what are superficially homogeneous formations, 
but upon analysis these are seen to contain consocies. The latter, though 
more uniform than formations, break up into groups, each of which still 
shows a characteristic heterogeneity arising from the varying number and 
arrangement of its constituent species. 

343. Causes. The primary cause of alternation is physical asymmetry, 
which is everywhere present within the symmetrical areas which produce 
zones. This is influenced so strongly, however, by migration and plant 
competition (phyteris) that the consideration of this subject will gain in 
clearness if these are treated as separate causes. The essential relation be- 
tween them must not be lost sight of, however. Migration carries dissemi- 
nules into all, or only some of the different areas of a formation, or into 
different formations, with little respect to the physical nature of these. The 
physical character of these asymmetrical areas determines that some of 
these plants shall be established in one series of places, and some in another, 
while the competition between the individuals in the various areas determines 
the numerical value of each species as well as its persistence. These three 
causes are invariably present in the production of alternating areas, and 
originally, i. e., in new or denuded soils, the sequence is constant, viz., migra- 
tion, ecesis in asymmetrical areas, and competition. 



Elements, F. E. The Development and Structure of Vegetation. Rep. Bot. Surv. 
Nebr., 7:16:?. 1904. 



ALTERNATION 285 

With respect to the different portions of an asymmetrical area, migration 
will have one of three effects : ( I ) it will carry disseminules into both 
favorable and unfavorable areas, (2) into favorable ones only, or (3) into 
unfavorable ones alone. From the radial nature of migration, the first case is 
far the most frequent ; it is typical of sporostrotes, and the highly specialized 
spermatostrotes and carpostrotes. The effect of migration is uniform here, 
and alternation arises in consequence of the selective power of ecesis. It is 
evident that migration does not have an even indirect effect, when the dis- 
seminules are carried into none but unfavorable situations. Where the move- 
ment is into favorable places alone, alternation is the immediate result. The 
intermittent operation of migration and the presence of barriers are re- 
sponsible for the absence of plants in situations favorable to them, and in 
consequence bring about a certain alternation between corresponding species. 

The selective operation of physical factors upon the disseminules carried 
into the different parts of an asymmetrical area is the usual cause of alterna- 
tion. Asymmetry alone is universal within the more conspicuous structures 
termed zones, down to the smallest areas which a group of plants can occupy. 
The difference between contiguous areas, particularly within the same habitat, 
is often small. It sometimes seems inefficient in the initial stages of a suc- 
cession when a single species is present, but even in extreme cases its effect 
will be recognizable in the size and density of the individuals. Asymmetry 
is clearly evident in vegetation where two symmetrical series cross each other, 
or when a symmetry is interrupted by barrier-like elevations or depressions. 
Within formations, it arises from differences, often very slight, in slope, 
exposure, elevation, from irregularities of surface, differences in soil struc- 
ture, or composition, in the amount of cover, and in the reactions of the living 
plants. At the last point, it is in direct connection with plant competition. 

344. Competition. Much uncertainty, as well as diversity of opinion, 
seems still to exist in regard to the precise nature of the competition between 
plants that occupy the same area. It has long been admitted that the 
phrase, "struggle for existence," is true of this relation only in the most 
figurative sense, but the feeling still prevails that, since plants live in asso- 
ciations, there must be something mysterious and vitalistic in their relation. 
No one has been able to discover anything of this nature, but nevertheless 
the impression remains. Such a direct relation exists only between 
parasites, epiphytes, and lianes, and the plants which serve to nourish 
or support them. In the case of plants growing on the same stratum, actual 
competition between plant and plant does not occur. One individual can 
affect another only in as much as it changes the physical factors that in- 
fluence the latter. Competition is a question of the reaction of a plant upon 
the physical factors which encompass it, and of the effect of these modified 



286 THE FORMATION 

factors upon the adjacent plants. In the exact sense, two plants do not com- 
pete with each other as long as the water-content and nutrition, the heat 
and light are in excess of the needs of both. The moment, however, that 
the roots of one enter the area from which the other draws its water supply, 
or the foliage of one begins to overshade the leaves of the other, the reaction 
of the former modifies unfavorably the factors controlling the latter, and 
competition is at once initiated. The same relation exists throughout tfte 
process; the stronger, taller, the more branched, or the better rooted plant 
reacts upon the habitat, and the latter immediately exerts an unfavorable 
effect upon the weaker, shorter, less branched, or more poorly rooted plant. 
This action of plant upon habitat and of habitat upon plant is cumulative, 
however. An increase in the leaf surface of a plant not merely reduces the 
amount of light and heat available for the plant near it or beneath it, but 
it also renders necessary the absorption of more water and other nutritive 
material, and correspondingly decreases the amount available. The inevit- 
able result is that the successful individual prospers more and more, while 
the less successful one loses ground in the same degree. As a consequence, 
the latter disappears entirely, or it is handicapped to such an extent that it 
fails to produce seeds, or these are reduced in number or vitality. 

Competition in vegetation furnishes few instances as simple as the above, 
but this will serve to make clear the simplest case of ordinary competition, 
i. e., that in which the individuals belong to a single species. The various 
individuals of one species which grow together in a patch show relatively 
slight differences, in height, width, leaf expanse, or root surface. Still, some 
will have the largest surfaces for the impact of water, heat, and light, while 
others will have the smallest; the majority, perhaps, will occupy different 
places between the extremes. The former will receive more than their share 
of one or more factors. The reaction thus produced will operate upon the 
plants subject to it inversely as the amount of surface impinged upon. The 
usual expression of such competition is seen in the great variation in height, 
branching, etc., of the different individuals, and in the inability of many to 
produce flowers. This is particularly true of annuals, and of perennials of 
the same generation. In the competition between parents and offspring of 
the same perennial species, the former usually have so much the advantage 
that the younger plants are often unable to thrive or even germinate, and 
disappear, leaving a free space beneath and about the stronger parents. 
This illustrates the primary law of competition, viz., that this is closest 
when the individuals are most similar. Similar individuals make nearly the 
same demands upon the habitat, and adjust themselves least readily to their 
mutual reactions. The more unlike plants are, the greater the difference in 
their needs, and some are able to adjust themselves to the reactions of others 
with little or no disadvantage. 



ALTERNATION 287 

In accordance with the above principles, the competition is closer between 
species of like form than between those that are dissimilar. This similarity 
must be one of vegetation or habitat form, not one of systematic position. 
The latter is in fact of no significance, except where there is a certain cor- 
respondence between the two. Leaf, stem, and root characters determine the 
outcome, and those species most alike in these features will be in close com- 
petition, regardless of their taxonomic similarity or dissimilarity. This is as 
conclusive of the competition between the species of the same genus as it is 
between those belonging to genera of widely separated families. From this 
may be deduced a second principle of competition, viz., the closeness of the 
competition between the individuals of different species varies directly with 
their similarity in vegetation or habitat form. This principle is of primary 
importance in the competition which arises between occupants and invaders 
in the different stages of succession. Those invading species that show the 
greatest resemblance to occupants in leaf, stem, and root form experience the 
greatest difficulty in establishing themselves. The species, on the contrary, 
which are so unlike the occupants that they come in at a clear advantage or 
disadvantage, establish themselves readily, in the one case as a result of the 
reaction, in the other by taking a subordinate position. This principle lies 
at the base of the changes in succession which give a peculiar stamp to each 
stage. A reaction sufficient to bring about the disappearance of one stage 
can be produced only by the entrance of invaders so different in form as to 
materially or entirely change the impress of the formation. Stabilization 
results when the entrance of invaders of such form as to exert an efficient 
reaction is no longer possible. In forests, while many vegetation forms can 
still enter, none of these produce a reaction sufficient to place the trees at a 
disadvantage, and the ultimate forest stage, though it may change in compo- 
sition, can not be displaced by another. 

It is obvious that the vegetation forms and habitat forms of associated 
species are of fundamental importance in determining the course and result 
of competition. Identity of vegetation form regularly produces close com- 
petition, and the consequent numerical reduction or disappearance of one or 
more species. Dissimilarity, on the other hand, tends to eliminate competi- 
tion, and to preserve the advantage of the superior form. Species of trees 
compete sharply with each other when found together; the same is true of 
shrubs, or rosettes, etc. The relation of the shrubs to the trees, or of the 
rosettes to the shrubs of a formation is one of subordination rather than of 
competition. The matter of height and width often enters here also to such 
a degree that the tallest herbs compete with the bushes and shrubs, and 
rosettes with mats or grasses. The amount and disposition of the leaf sur- 
face are decisive factors in the competition between species of the same 



288 THE FORMATION 

vegetation form, in so far as this is governed by light. In those plants in 
which the leaves are usually erect, notably the grasses and sedges, the com- 
petition between the aerial parts is relatively slight, and the result is de- 
termined by the reactions of the underground stems and roots. 

The position of the competing individuals is of the greatest importance. 
The distance between the plants affects directly the degree of competition, 
while their arrangement, whether in groups according to species or singly, 
exerts a marked influence by determining that the contest shall be between 
like forms, or unlike forms. Position is controlled primarily by the relation 
existing between seed-production and dissemination. It is of course in- 
fluenced in large measure by the initial position taken by the invaders into a 
nudate area, but this is itself a result of the same phenomena. The in- 
dividuals of species with great seed-production and little or no mobility 
usually occur in dense stands. In these, the competition is fierce, for the 
two reasons of similarity and density, and the result is that the plants fall 
far below the normal in height and width. This is an extreme example of 
the group arrangement. When the seed-production is small, the mobility 
may be great or little without seriously affecting the result. The individuals 
of a species of this kind will be scattered among those of other species, and 
the closeness of competition will depend largely upon the similarity existing 
between the two. The arrangement in such cases is sparse. A species with 
great seed-production and great mobility usually shows both kinds of ar- 
rangement, the position of the individuals and the competition between them 
varying accordingly. This is due to the intermittent action of distributing 
agents, making it possible for the seeds to fall directly to the ground during 
the times that winds, etc., are absent. The three types of arrangement indi- 
cated above are termed gregarious, copious, and gregario-copious. They 
furnish the basis for the investigation of abundance which deals essentially 
with the number and arrangement of the individuals of competing species. 
The effect of distance, i. e., the interval between individuals, upon competi- 
tion is fundamental. The competition increases as the interval diminishes, 
and the reverse. 

The view here advanced, i. e., that competition is purely physical in nature, 
renders untenable the current conceptions of vegetation pressure, occupation, 
etc. Masses of vegetation are thought to force the weaker species toward 
the edge, thus initiating an outward or forward pressure. As has been 
shown above, no such phenomenon occurs in vegetation. This movement 
is nothing but simple migration, followed by ecesis, and has no connection 
with "weaker" species, or the development of a vital pressure. The direc- 
tion taken by the migrating disseminules is essentially indeterminate. Mi- 
gration seems to be outward, or away from the mass, merely because the 



. ALTERNATION 289 

ecesis is greater at the edge, where the increased dissimilarity between plant 
forms diminishes the competition. The actual movement is outward, but it 
takes place through the normal operation of competition. In this connection, 
it should be pointed out that the common view that plants require room is 
inexact, if not erroneous. This is difficult of proof, as it is impossible to dis- 
tinguish room as such from the factors normally present, light, heat, water, 
and nutrient salts, but it seems obvious that the available amounts of these 
will determine the space occupied by a plant, irrespective of the room adja- 
cent plants may allow it. The explanation of competition upon physical 
grounds likewise invalidates the view that plants possess spheres of influence 
other than the areas within which they exert a demonstrable reaction upon 
the physical factors present. 

Competition plays a very important role in alternation. It produces minor 
examples of alternation in the physical units of an asymmetrical series. Its 
greatest influence, however, is exerted in modifying the effects of asym- 
metry. The reaction of occupants emphasizes or reduces the effect of asym- 
metry, and has a corresponding action upon alternation. This result of com- 
petition is typical of succession, in which the sequence of stages arises from 
the interaction of occupant and invader. 

345. Kinds of alternation. Alternation involves two ideas, viz., the al- 
ternation of different species or formations with each other, and the alterna- 
tion of the same species or formation in similar but separate situations. This 
is the evident result of asymmetry, in response to which contiguous areas 
are dissimilar and remote ones often similar. Individuals of the same species 
or examples of the same formation may be said to alternate between two or 
more similar situations, while different species or formations are said to al- 
ternate with each other, occurring usually in situations different in char- 
acter. From the nature of alternation, the two phenomena are invariably 
found together. 

It is possible to distinguish three kinds of alternation: (i) of a formation, 
consocies, layer, facies, or species in similar situations; (2) of similar or 
corresponding formations, species, etc., in similar situations; (3) of facies 
and other species with respect to number. The last two are merely varia- 
tions of the first, arising out of slight differences in the physical factors of 
the alternating areas, the adjacent flora, or the course of competition. The 
alternation of different examples of the same formation is a significant fea- 
ture of greatly diversified areas, such as mountains. It is naturally much 
less characteristic of lands physiographically more uniform. A xerophytic 
formation will alternate from ridge to ridge, a mesophytic formation between 
the intermediate valleys ; aquatic vegetation will alternate from pond to pond, 



29O THE FORMATION 

or stream to stream. The appearance of new or denuded soils upon which 
successions establish themselves is the most important cause of the alterna- 
tion of formations. The weathering of rocks in different areas of the same 
region produces in each a sequence of similar or identical formations. The 
same statement is true in general of other causes of succession, such as ero- 
sion, flooding, burning, cultivation, etc., wherever they operate upon areas 
physically similar and surrounded by the same type of vegetation. The areas 
of more or less heterogeneous formations characterized by major physical 
differences are occupied by consocies. In an extensive formation, the 
same consocies alternates from one to another of these areas that are simi- 
lar. When the formation is interrupted and occurs here and there in sep- 
arate examples, a consocies often alternates from one to another of these. 
A consocies regularly derives its character from the fact that one or more 
of the facies of the formation is more intimately connected with certain 
areas of the ]atter than with others. This explains why the alternations of 
consocies and facies are usually identical. Layers sometimes alternate be- 
tween different examples of the same forest or thicket formation, when they 
are suppressed in some by the diffuseness of the light. 

The alternation of species is a typical feature of formations; it is absent 
only in those rare cases where the latter consist of a single species. The 
areas of a habitat which show 7 minor physical or historical (i. e., competi- 
tive) differences are occupied by groups of individuals belonging to one or 
more species responsive to these differences. Each of these groups will recur 
in all areas essentially similar, the intervals being occupied of course by 
slightly different groups. Such groups are constituted by gregarious or copi- 
ous species of restricted adjustability. Sparse plants likewise alternate, but 
they necessarily play a much less conspicuous part. In habitats not too heter- 
ogeneous, a large number of species are sufficiently adjustable to the slight 
differences so that they occur throughout the formation. Often, to be sure, 
they show a characteristic response, expressed in the size or number. This is 
illustrated by the facies and many of the principal species of the prairie for- 
mation. Festuca, Koelera, Panicum, and Andropogon occur throughout, ex- 
cept in the moist ravines which are practically meadows. Astragalus, 
Psoralea, Erigeron, and Aster grow everywhere on slopes and crests, but 
they are much more abundant in certain situations. Other plants, Lomatium, 
Meriolix, Anemone, Pcntsiemon, etc., recur in similar or identical situations 
upon different hills. Lomatium alternates between sandy or sandstone crests, 
Meriolix and Pentstemon occur together upon dry upper slopes, while Ane- 
mone alternates between dry slopes and crests. 

Owing to the accidents of migration and competition, similar areas within 
a habitat are not occupied by the same species, or group of species. A spe- 



ALTERNATION 



29I 



cies found in one area will be replaced in another by a different one of the 
same or a different genus. The controlling factors of the area render imper- 
ative an essential identity of vegetation and habitat form, though in system- 
atic position the plants may be very diverse. Such genera and species may 
be termed corresponding. The relation between such plants is essentially 
alternation ; it should, perhaps, be distinguished from alternation proper as 
corresponsive. The prairie formation furnishes a good example of this on 
exposed sandy crests, upon which Lomatium, Comandra, and Pentstemon 
alternate. Formations exhibit a similar correspondence. 




Fig. 74. Numerical alternation of Pinus and Pseudotsuga upon east 
and west slopes. 

All species that alternate show a variation in abundance from one area to 
another. Frequently, the difference is slight, and may be ignored, except in 
determining abundance. Very often, however, the variation is so great that 
a facies may be reduced, numerically, to the rank of a principal species, or 
one of the latter to a secondary species. This phenomenon is distinguished 
as numerical alternation. It arises from the fact that the similar areas are 
sufficiently different to affect the abundance, without producing complete 
suppression. It is probable that this result is due almost entirely to compe- 
tition. Astragalus crassicarpus grows on all the slopes of the prairie forma- 
tion, but on some it has the abundance of a facies, while on others it is repre- 



292 THE FORMATION 

sented by a few scattered individuals. This difference is much more striking 
in separate examples of the same formation, particularly when a normal 
facies is reduced to the numerical value of a secondary species. This is a 
matter of great importance in the study of formations, for it has doubtless 
often resulted in mistaking a consocies for a formation. 

Alternation furnishes the logical basis for what may be called comparative 
phytogeography. The latter is of much broader scope than the old subject 
of geographical distribution, for it treats not only of the distribution of for- 
mations and associations as well as of species, but it also seeks to explain 
this by means of principles drawn from the relation between habitat and 
vegetation. When the latter come to be fully based upon physical factor in- 
vestigations, and upon the effects of migration and competition as shown in 
alternation, the comparative study of formations will represent the highest 
type of phytogeographical activity. 

THE FORMATION IN DETAIL 

346. The rank of the formation. There have been as many different 
opinions in regard to the application of the term formation as there are con- 
cerning the group which is to be called a species. In taxonomy, however, the 
concept of the species is purely arbitrary, and agreement can not be hoped 
for. In vegetation, en the contrary, the connection between formation and 
habitat is so close that any application of the term to a division greater or 
smaller than the habitat is both illogical and unfortunate. As effect and 
cause, it is inevitable that the unit of the vegetative covering, the formation, 
should correspond to the unit of the earth's surface, the habitat. This places 
the formation upon a basis which can be accurately determined. It is im- 
perative, however, to have a clear understanding of what constitutes the 
difference between, habitats. A society is in entire correspondence with the 
physical factors of its area, and the same is true of the - vegetation of a prov- 
ince. Nevertheless, many societies usually occur in a single habitat, and a 
province contains many habitats. The final test of a habitat is an efficient 
difference in one or more of the direct factors, water-content, humidity, and 
light, by virtue of which the plant covering differs in structure and in spe- 
cies from the areas contiguous to it. A balsam-spruce forest shows within 
itself certain differences of physical factors and of structure. The water- 
content will range from 20—25 per cent, and the light from .02-003. One 
portion may consist chiefly of Pseudot'suga mucronata, another of Picea 
engelmannii, and a third of Picea parry ana, or these species may be inter- 
mingled. If, however, this forest is compared with the gravel slide, which 
touches it on one side, and the meadow thicket, which meets it on another, 



IN DETAIL 293 

the physical factors and the species both demonstrate that it is the forest, 
and not its parts, which corresponds to a distinct physical entity, the hab- 
itat. This test of a formation is superfluous in a great many cases, where 
the physiognomy of the contiguous areas is conclusive evidence of their 
difference. It is evident also that remote regions which are floristically dis- 
tinct, such as the prairies and the steppes, may possess areas physically al- 
most identical and yet be covered by different formations. This point is 
further discussed under classification. 

The existing confusion in the matter of formations is due to two causes. 
The first arises from the fact that much ecological work has been hasty. 
Little or no attention has been given to development, and in consequence 
rudimentary and transitory stages of succession have often been described 
as formations. Mixed areas in particular have caused trouble. In the sec- 
ond place, there has been a marked tendency to minimize the need of thor- 
oughness and training by calling every slightly different area a formation. 
A failure to recognize the primary value of alternation has also contributed 
materially to this. Alternating facies, and principal species, when separated 
from each other, have often been mistaken for formations. This is a danger 
that must be fully appreciated and guarded against. In practically all re- 
gions, the same formation is represented by numerous scattered areas, all 
showing greater or less differences arising from alternation. This is espe- 
cially true of thickly populated regions where virgin areas are rare. The 
fact that twenty-five miles intervene to-day between two small stretches of 
primitive prairie is permitted to unduly emphasize their differences. It re- 
quires the study of a number of such examples to counteract this tendency, 
and to cause one to see clearly that they must have been at one time merely 
so many bits of the prairie formation. 

In this connection, the lichen and moss groups which are found on rocks 
constitute an interesting problem. It is clear that Peltigera and Cladonia, 
which grow on the forest floor, and Evernia, Ramalina, and Physcia, which 
are found on the trees, are merely constituent species of the forest forma- 
tion. The same is true of Cladonia, Urceolaria, and Parmelia, which are 
found among the sedges and grasses of alpine meadows. The physical con- 
ditions are essentially those of the formation, and the lichens themselves 
are more or less peculiar to it. This is particularly true of the forest, in 
which the two strata, bark and moist shaded soil, are present because of the 
trees. In the case of granitic rocks, the circumstances are very different. 
The species of lichens found on the rocks are not peculiar to the formation, 
but they also occur elsewhere. In the forest, Parmelia, Placodium, Physcia, 
Rinodina, Urceolaria, Lecanora, Lecidea, etc., occur on the rocks. In the 
alpine meadows, the rock groups are composed of Parmelia, Gyrophora, 



294 



THE FORMATION 



Cetraria, Acarospora, Lecanora, I.ecidea, Buellia, etc. The stratum itself is 
physically very . different and constitutes a distinct habitat. These groups 
are really small formations, which are quite distinct from the surrounding 
forest or meadow. This is proven conclusively in many places in the moun- 
tains where areas of the characteristic lichen formations of cliffs are carried 
by the fall of rock fragments into forest and meadow, where they persist 
without modification. This also shows clearly that the groups on scattered 
rocks in the same area are to be regarded as examples of the same cliff for- 
mation, except where the differences are evidently to be ascribed to develop- 




Fig. 75. Relict lichen formation in a spruce forest, invaded by rock 
mosses. 

ment and not to alternation. Where these rock formations can not be traced 
to cliffs or magmata with certainty, they must be considered as antedating 
the vegetation in which they occur. Often, indeed, especially in igneous 
areas, they are relicts of the initial stage of a primary succession. Finally, 
they prove their independence of the forest or meadow formation by initiat- 
ing a distinct succession within these. Crustaceous groups or formations 
yield to foliose ones, and these in turn give way to formations of mosses, 
particularly in the forest where the effect of the diffuse light is felt. From 
the above, the following rule of f ormational limitation is obtained : any area, 
which shows an essential difference in physical character, composition, or 
development from the surrounding formation is a distinct formation. 



IN DETAIL 



295 



347. The parts of a formation. All the parts which make up the struc- 
ture of a formation are directly referable to zonation and alternation, alone or 
together, or to the interaction of the two. The principles which underlie 
this have already been discussed under the phenomena concerned. It is 
necessary to point out further that the structure may be produced in several 
ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as 
primary and alternation as secondary, (4) by primary alternation and sec- 
ondary zonation, (5) by the interaction of the two, as in layered formations. 
Though all these methods occur, the first two are relatively rare, and the 




Fig. 76. Early (prior) aspect of the alpine meadow formation (Ca- 
rex-Campanula-coryphium) , characterized by Rydbergia grandiflora. 



resulting structure comparatively imperfect. The typical structure of for- 
mations can best be made clear by the consideration of a prairie which be- 
longs to the fourth group, and a forest which represents the last. 

The major divisions of prairie and forest formations are regularly due to 
alternation. There is an inherent tendency to the segregation of facies, 
arising out of physical or historical reasons, or from a combination of both. 
Not all formations show this, but it is characteristic of the great majority 
of them. The primary areas which thus arise have been called associations: 
they are naturally subordinate to the formation. To avoid the confusion 
which inevitably results from using the word association in two different 



2g6 



THE FORMATION 



senses, it is proposed to term this primary division of the formation, a con- 
sociation, or better, a consocies. This term is applied only to an area char- 
acterized by a facies, or less frequently, by two or more facies uniformly 
commingled. The consocies of grassland are determined by grasses, those 
of forests by trees, etc. From the different position of the facies in these 
two types of vegetation such areas are readily seen at all times in the forest, 
but they are often concealed in grassland by the tall-growing principal spe- 
cies of the various aspects. When definite consocies are present, they are 
often found to mingle where they touch, producing miniature transition 
areas, and, very rarely, they sometimes leave gaps in which no facies appears. 





- ■•-■ 






mm 




t^vV'"'.^"-.">i 




: -M ?% ■ ■ A 
h A J0 '„ 


I,-,- ■ ' , 


'#"" 


. * J ": ■■ 'S'i'. • . v 




















''■':■-:■'- 


<%' 





Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized 
by Campanula petiolata, Rydbergia in fruit. 



The seasonal changes of a formation, which are called aspects, are indi- 
cated by changes in composition or structure, which ordinarily correspond to 
the three seasons, spring, summer, and autumn. The latter affect the facies 
relatively little, especially those of woody vegetation, but they influence the 
principal species profoundly, causing a grouping typical of each aspect. 
For these areas controlled by principal species, but changing from aspect to 
aspect, the term society is proposed. They are prominent features of the 
majority of herbaceous formations, where they are often more striking than 
the facies. In forests, they occur in the shrubby and herbaceous layers, and 



IN DETAIL 



297 



are consequently much less conspicuous than the facies. A close inspection 
of the societies formed by principal species shows that they are far from 
uniform. Since they usually fail to exhibit distinct parts, it becomes neces- 
sary to approach the question of their structure from a new standpoint. 
Such is afforded by aggregation, which yields the simplest group in vegeta- 
tion, i. e., that of parent and offspring. This is so exactly a family in the 
ordinary sense that there seems to be ample warrant for violating a canon of 
terminology by using the word for this group, in spite of its very different 
application in taxonomy. It has already been shown that aggregation fur- 
ther produces a grouping of families, which may properly be called a corn- 




Fig. 78. Calthetum ( Caltha leptosepala) , a consocies of the alpine bog 
formation. 



munity. As they are used here, family and community become equally ap- 
plicable to the association of plants, animals, or man. Both families and 
communities occur regularly in each society of the formation, and they repre- 
sent its two structures. In some cases, all the families are grouped in com- 
munities, two or more of which then form the society. Very frequently, 
however, families occur singly, without reference to a community, and the 
two then constitute independent parts of the same area. This is typically 
the case wherever gregarious species are present, since these are merely 
family groups produced by aggregation. 



298 



THE FORMATION 



Objection may be made that this analysis of formational structure has 
been carried too far, and that some of the structures recognized are mere 
interpretations, and not actual facts. Such a criticism will not come from 
one who has got beyond the superficial study of formations,- for he will at 
once recognize that certain probable features of structure have not been con- 
sidered. On the other hand, the ecologist or the botanist who has not made 
a careful investigation from the standpoints of development and structure 
will naturally refrain from expressing an opinion, until he has obtained an 
acquaintance at first hand with the facts. Over-refinement is the usual pen- 




Fig, 
tion. 



79. Iridile (Iris missouriensis) , a society of the aspen forma- 



alty of intensive work. The unbiased investigator, however, will not be mis- 
led by the suddenness with which new concepts appear. It seems plausible 
that the structure of a formation, if not as definite, is at least nearly as com- 
plex as that of an individual plant. Few botanists will insist that the re- 
finement of tissues and tissue systems has been carried further than the 
differentiation of the plant warrants. Yet, if these had been defined within 
a period of a few years rather than slowly recognized during more than a 
century, they would have been called seriously in question. As a matter of 
fact, the consocies, under the term association, and the society, under various 
names, have been recognized by ecologists for several years. They are defi- 



IN DETAIL 299 

nite phenomena of alternation which can be found anywhere. The family 
and the community, though the latter is less distinct in outline, are equally 
valid structures, the proof of which anyone can obtain by thorough methods 
of study. 

348. Nomenclature of the divisions. The suffix -etum is used to desig- 
nate a consocies of a formation, e. g., Picetum, Caricetum, etc. When two or 
more, species characterize the area, the most important, or more rarely, the 
two are used. The termination used to designate a society is -He, as Asterile, 
Sedile, Rosile. The suffix which denotes the community is -are, and. for the 
family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon, etc. Layers 
are indicated by the affix -amim, as Opulasteranum, Verbesina-Rudbecki- 
anufitj etc. It is evident that these suffixes, like the terms to which they re- 
fer, must be used always for the proper divisions if they are to have any 
value at all. There has been a marked tendency, for example, to use -etum 
in connection with the names of groups of very different rank. It is hardly 
necessary to point out that such a practice does not promote clearness. The 
following tabular statement will illustrate the application of both terms and 
suffixes : 

Picea-Pseudotsuga-hylium formation {-turn) Paronychia-Silene-chalicium 

Picetum consocies (-etum) Paronychietum 

Opulaster-Ribesanum layer (-ami m) 

Opulasterile society (-He) Androsacile 

Thalictrare community (-are) Festucare 

Pirolon family (-on) Arenarion 

349. The investigation of a particular formation. A comprehensive and 
thorough study of a formation should be based upon as many examples of it 
as are accessible. The example which is at once the most typical and the 
most accessible is made the base area. This plan saves time and energy, re- 
duces the number of instruments that are absolutely necessary, and estab- 
lishes a common basis for comparison. The inquiry should be made along 
four lines, all fundamental to a proper knowledge of the formation. These 
lines are: (1) the determination of the factors of the habitat, (2) a quadrat 
and a transect study of the structure of the formation, (3) a similar inves- 
tigation of development, (4) a floristic study of the contiguous formation, 
with special reference to migration. The sequence indicated has proven to 
be the most satisfactory, and is to be regarded as all but absolutely essential. 
Naturally, this applies only to the order in which the various lines are to be 
taken up, as they are carried on together when the work is fully under way. 
Since instrument and quadrat methods have already been given in detail, it 



300 



THE FORMATION 



is unnecessary that they be repeated. Similarly, the questions which pertain 
to structure and development and to the surrounding vegetation are con- 
sidered in detail in the pages which precede. 











4^t 








fife ftST - pB*^ "• '• .^Bj 


X- ■' "'-1 

J 


' '. . '.' • 


ft 




'■;-.,■ ■'■■■ - 


r. - 
•> 


'- 








HH 




- ; r 




1 w 



Fig. 80. Eritrichiare (Eritrichium aretioides) , a community of the 
alpine meadow formation. 



CLASSIFICATION AND RELATIONSHIP 



350. Bases. Formations may be grouped with reference to habitat or 
kind, development or position. Classification upon the basis of habitat 
places together formations which are similar in physiognomy and structure. 
Developmental classification is based upon the fact that the stages of a par- 
ticular succession are organically connected or related, though they are nor- 
mally different in both physiognomy and structure. Grouping with respect 
to position is made solely upon occurrence in the same division of vegeta- 
tion. The formations thus brought together usually possess neither similar- 
ity of kind or structure, nor do they have any necessary developmental con- 
nection. Habitat and developmental classification are of fundamental value; 
regional arrangement is more superficial in character. All serve, however, 
to emphasize different relations, and, while the developmental system ex- 
presses the most, they should all be used to exhibit the vegetation of a region, 
province, or zone. 



CLASSIFICATION AND RELATIONSHIP 



301 



351. Habitat classification. In arranging formations with reference to 
habitats, the direct factors, water and light, can alone be used to advantage. 
Such a system is fundamental, because it is founded upon similarity of hab- 
itat and of structure. Proposed groupings based upon nutrition-content, or 
upon the division of factors into climatic and edaphic, have elsewhere 1 been 
shown to be altogether of secondary importance, if not actually erroneous. 
The basis of the habitat grouping is water-content, which is supplemented 
by light whenever the factor is decisive. The primary divisions thus ob- 
tained are water, forest, grassland, and desert, which are characterized re- 







SSfflPfiSKa 






..*».. ~ ■ 






:»«*»-■■• «f» a®B 




$/ "^*r 


->;h ' 


m§B&£Ri$ * 


* 






. 










Ifllflim. '•' 


5 








{fjf -Wr »\(£T • M_ s 








""% 








•HA "4 




?§^n\y6£^ 




jffSisH 


- 








ill 




* x < > i 




?lPi 


■*; _ 




■ S y -' : ~''^m* 


*i-'i*--..Ci 








-, ; ^\r- 




.-.v.:;: 


^•f^ 

&££■**; 


... .. ..... . 








• ■ «; "^ T'x, ' '■ "v'.T"^ 


v \§§y 



Fig. 81. Pachylophon (Pachylophus caespitosus), a family of the 
gravel slide formation. 

spectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, 
and xerophytes respectively. Within these, formations are arranged ac- 
cording to the type of habitat, i. e., pond, meadow, forest, dune, etc. These 
divisions comprise all formations which belong to the type by virtue of their 
physiognomy and structure. Such formations differ from each other very 
considerably or completely in the matter of floristic, i. e., component species, 
but they still belong to the same type. A dune formation in the interior and 
one on the coast may not have a single species in common, and yet they are 
essentially alike in habitat, development, and structure. 



1 Clements, F. E. The Development and Structure of Vegetation, 24, 27. 1904. 



302 



THE FORMATION 



352. Nomenclature. The names of formations are taken from the hab- 
itats which they occupy. Each formation should have a vernacular and a 
scientific name. The latter is especially important since it ensures brevity 
and uniformity, and obviates the obscurity and confusion that arise from 
vernacular terms in many tongues. Scientific names have been made uni- 
formly from Greek words of proper meaning by the addition of the suffix 
-ium (ttov), which denotes place. 1 The following list gives the English and 
the scientific name of the various habitats, and their corresponding forma- 
tions, and indicates the primary divisions into which these fall. 



I. Hydrophytia: water plant for- 
mation's 

1. ocean: oceanium : oce- 

anad, 2 oceanophilous, 
etc. 

2. sea : thalassium 

surface of the sea: 

pelagium 
deep sea: pontium 

3. lake : limnium, limnad 

4. pond, pool, tiphium, tip- 

had 

5. stagnant water: stas- 

ium : stasad 

6. salt marsh : limnodium, 

limnodad 

7. fresh marsh : helium 

8. wet meadow : telmatium 

9. river : potamium 

10. creek, rhoium 

11. brook: namatium 

12. torrent: rhyacium 

13. spring: crenium 

14. warm spring: thermium 

15. ditch: taphrium 

16. sewer : laurium 



17. swamp forest: helohy- 

lium 

18. swamp open woodland : 

helodium 

19. meadow thicket: helo- 

drium 

20. bank: ochthium 

rock bank : petroch- 

thium 
sand bank : ammoch- 

thium 
mud bank: pelochthium 

21. rocky seashore: actium 

22. sandy seashore : agium 

23. sandbar: cheradium 

24. tank: phretium 

II. Mesophytia : middle plant for- 
mations 
a. Sciophytia : shade plant 
formations 
26. forest: hylium 
2J. grove : alsium 

28. orchard : dendrium 

29. canyon: ancium 

30. open woodland : orga- 

dium 



Elements, F. E. A System of Nomenclature for Phytogeography. Engler Jahrb., 
31:b70:l. 1902. 

a The terms, oceanad, hylad, poad, eremad, etc., are proposed in place of ocean- 
ophyte, hylophyte, etc. They are much shorter and make consistent groups under the 
general term, ecad., i. e., habitat form. 



CLASSIFICATION AND RELATIONSHIP 



303 



31. thicket: lochmium 


45- 


b. Heliophytia : sun plant for- 


46. 


mations 


47* 


32. meadow : poium 


48. 


33. pasture : nomium 


49. 


34. culture land : agrium 


50. 


35. waste place : chledium 


5i. 


III. Xerophytia : dry plant forma- 


52. 


tions 


53- 


36. desert: eremium 


54. 


37. sand-hills, sandy plain: 


55- 


amathium 


56. 


38. prairie, plains : psilium 




39. dry, open woodland: 


57. 


hylodium 


58. 


40. dry thicket : driodium 




41. dry forest: xerohylium 


59- 


42. gravel slide : chalicium 


60. 


43. sandbar: syrtidium 


61. 


44. sand draw : enaulium 





blowout : anemium 
strand : psamathium 
dune : thinium 
badlands : tirium 
hill, ridge : lophium 
cliff : cremnium 
rock field : phellium 
boulder field : petrodium 
rock, stone : petrium 
humus marsh : oxodium 
alkali area: drimium 
heath, dry meadow : 

xeropoium 
moor : sterrhium 
alpine meadow : cory- 

phium 
polar barrens : crymium 
snow : chionium 
wastes : chersium 



Particular formations are indicated by means of floristic distinctions. 
Thus, Populus-hylium is the aspen forest as distinguished from the Picea- 
Pseudotsuga-hylium. or the balsam-spruce forest; and the Bulb His- psilium, 
or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama- 
bluestem prairie. Similarly, the aspen formation of the Old World and of 
the New may be distinguished as Populus-tremula-hylium and Populus- 
tremuloides-hylium, respectively. In all formational names, the facies alone 
should be used. Frequently, a single facies will suffice for clearness. As a 
rule, however, the two most important facies should be employed; in rare 
cases only is it necessary to use the names of three. When it is desirable to 
refer to two or more examples of the same formation, a geographical term is 
added, e. g., (1) Populus-hylium {Crystal Park), (2) Populus-hylium 
{Cabin Canyon). 



353. Developmental classification. This is based upon succession as the 
record of development. Upon the basis of development, all the formations 
which belong to the same succession are classed together. They are ar- 
ranged within each group in the sequence found in the particular succession. 
From its nature, developmental classification is of primary importance in 
exhibiting the history of vegetational changes. It has less value than the 



304 THE FORMATION 

habitat system for summarizing the essential structure of a vegetation, in- 
asmuch as it places the emphasis upon historical rather than structural fea- 
tures. It is evident that both deal with the same formations, and that the 
difference is merely one of viewpoint. The habitat classification is simpler 
in that it considers only those formations actually on the ground, while de- 
velopment has regularly to take into account stages which have disappeared. 
The groups of the developmental system, and the arrangement of formations 
within them have already been indicated under the nomenclature of succes- 
sion (sections 326 and 327). 

354. Regional classification. The grouping of formations with respect 
to the divisions of vegetations is chiefly of geographical value. It indicates a 
certain general relationship, but its principal use is to summarize the struc- 
ture of the vegetative covering of a region. The arrangement of formations 
in the various divisions is made with reference to the outline of North 
American vegetation (section 341). This is naturally based upon the iden- 
tity of altitude and latitude zones. In the study of mountain countries, it is 
often desirable to group formations with reference to altitude alone. In 
this case, the grouping is based upon the following divisions: (1) bathyphy- 
tia, lowland plant formations; (2) mesiophytia, midland formations; 
(3) pediophytia, upland formations; (4) pagophytia, foothill formations; 
(5) orophytia, subalpine formations; (6) acrophytia, alpine formations; 
(7) chionophytia, niveai formations. 

355. Mixed formations. These are mixtures of two, rarely more, adja- 
cent formations, or of two consecutive stages of the same succession. Mixed 
formations are really transitions in space or in time between two distinct 
formations. Theoretically, they are to be referred to one or the other, ac- 
cording to the preponderance of species. Actually, however, they often 
persist in an intermediate condition for many years, and it becomes necessary 
to devote considerable attention to them. In some cases, there is good rea- 
son to think that the species of two contiguous formations have become per- 
manently associated, and thus constitute a new formation. This is often 
apparently true in succession, when the change from one stage to the next 
requires a long term of years, but it is really true only of the very rare cases 
in which a succession becomes stabilized in a transition stage. When the 
mixture is due to development, the formations concerned are often quite dis- 
similar, e. g., grassland and thicket, thicket and forest. If it is the result of 
position, the formations are usually similar, i. e., both are grassland, thicket, 
or forest, since the plants of the lower level are regularly assimilated or de- 
stroyed, when invasion occurs at two levels. The term mictium C/jclktcv, 



CLASSIFICATION AND RELATL 



305 



mixture) is here proposed for the designation of all mixed formations, 
whether they arise from succession or from juxtaposition. Thus, the Ment- 
zelia-Elymus-mictium is the transition between the Mentzelia-Pseudocymop- 
ierus-chaUcium and the Elymus-Muhlenbergia-chalicium. Similarly, the 
P pulus-Picea-mictium and. the Pinus-Pseudotsnga-mictium are transition 
stages in the development of the Picea-hylium. On the other hand, the And- 
ropogon-BulbiUs-mictium is a mixture produced by the mingling of two 
contiguous prairie formations. In the future development of this subject, it 
will probably become desirable to name mixed formations on the basis of 




Fig. 82. A mixed formation of aspens and spruces (P '0 pulus-Picea- 
mictium) , preceding the final spruce forest of a burn succession. 



origin, but at present this is unnecessary. Both in classification and in de- 
scription they should be considered between the formations which give rise 
to them, and this will at once indicate their origin. 

Puzzling cases of mixture resulting from position occur toward the limits 
of facies which occupy extensive areas. Bouteloua oligostachya, and And- 
■ropogon scoparms extend from the prairies through the sand-hills and plains, 
and into the foot-hills of the Rocky mountains. Their abundance at once 
raises a question as to the validity of the prairie, sand-hill, plain, and foot- 
hill formations. If these two grasses were controlling, and equally charac- 
teristic throughout, then the entire stretch would have to be regarded as a 



306 THE FORMATION 

single formation. Since they are often absent, or mixed with other facies of 
greater importance, they can not be considered the sole tests of the forma- 
tion. This view is reinforced by the fact that prairie, sand-hill, plains, and 
foot-hill all have their characteristic principal and secondary species, in ad- 
dition to facies that are more or less typical. In certain formations, doubt- 
less, Bouteloua and Andropogon are relicts, in others invaders, while in the 
formations actually constituted by them they are dominant. The final solu- 
tion of such problems is quite impossible, however, until the comparative 
study of large areas can be based upon the accurate detailed investigation of 
the component formations. 

Experimental Vegetation 

356. Scope and methods. The experimental study of the formation as a 
complex organism rests upon methods essentially similar to those discussed 
under experimental evolution. The scope of the two fields is practically the 
same, moreover, in that both deal with the experimental development of an 
organism and the structures that result. The actual problems are naturally 
very different, since the formation is a complex of individual plants, but the 
fundamental basis of habitat, function, and structure is common to both. 
However, the functions now to be considered are aggregation, invasion, com- 
petition, etc., and the structures, zones, consocies, societies, communities, and 
families. The latter may properly be regarded as adaptations called forth 
by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation 
to the physical factors of the habitat. As consequences of measured factors, 
formational adjustment and. adaptation must themselves be carefully meas- 
ured and recorded. For these purposes, the methods of quadrat and tran- 
sect, of chart, photograph, and formation herbarium are used. Invaluable 
as they are for any scientific inquiry into vegetation, such methods form the 
very foundation of experimental study in which accuracy is the first 
desideratum. 

It has already been shown that nature's own experiments in the production 
of new forms furnish the best material for experimental evolution. This 
statement is equally true of experimental vegetation. The formation of new 
habitats by weathering and transport, and the denuding of old ones, yield 
experimental plots of the greatest value. This is likewise the case in the 
great majority of formations, where invasion or competition is active. These 
are the phenomena that must be considered in any careful study of vegeta- 
tion, but in taking them up from the experimental standpoint, greater atten- 
tion must be paid to detail, and the changes must be followed closely for a 
longer time. The method that makes use of existing changes in vegetation 



METHOD OF NATURAL HABITATS ?> 7 

is designated the method of natural habitats. In contrast with this is the 
method of artificial habitats, in which the habitat itself is definitely modified, 
or a group of species actually transferred to a different habitat. Many prob- 
lems of vegetation can be attacked with greater success under control than 
in the field. This is particularly true of competition, in which results can be 
obtained most readily by means of the method of control habitats, as carried 
on in the plant house. 

METHOD OF NATURAL HABITATS 

357. Natural experiments. Every family as well as every community 
constitutes an experiment in competition; the same statement necessarily 
holds for the larger groups, society, consocies, and formation, which are com- 
posed of families and communities. The last also make it possible to study 
competition in two typical instances, viz., in the family, where the individuals 
are of one kind, and in the community, where they belong to two or more 
different species. The community, moreover, is a product of invasion, and it 
furnishes material for the study of this function, as well as for that of aggre- 
gation and competition. Practically every formation shows some invasion, 
but as a rule stable formations contain so few invaders that they are rela- 
tively unimportant in this connection. Invasion is most active in transition 
areas and in mixed formations, whether produced by juxtaposition or by 
succession, and its study in these places yields by far the largest number of 
valuable results. 

As typical complete invasion, a succession is the best of all natural ex- 
periments in aggregation, migration, ecesis, and competition. This is espe- 
cially true of the initial stages in which changes in the number and position 
are most readily followed. The methods used in studying successions have 
been given elsewhere. In addition, it should be pointed out that one of the 
first tasks in taking up the ecological investigation of a region is to make a 
careful search for all new and denuded areas, as well as for those in which 
succession is taking place. The phenomena in these areas can not be ex- 
plained until the habitats and formations have been worked over critically, 
but the facts must be collected at the earliest possible moment, since the 
stages of the succession are constantly changing, while the stable formations 
are not. 

METHOD OF ARTIFICIAL HABITATS 

358. Modification of habitat. As the final factors in ecesis and competi- 
tion, water, light, and temperature control the grouping of plants into vege- 
tation. An efficient change in one of these, or in all of them, brings about a 
visible adjustment in the structure of the plant group concerned. Modifica- 



308 THE FORMATION 

tions of water-content and light are readily produced in the field by drain- 
age, irrigation, shading, clearing, etc. In fact, all the changes of habitat 
indicated under experimental evolution serve equally well to initiate experi- 
ments in experimental vegetation ; indeed, the same experiment covers both 
fields. It is impracticable, however, to modify the temperature of a habitat 
without changing its water-content or light, and consequently the influence 
of temperature can not be determined through experiment by modification. 
The extent of the area modified should be as large as convenience will per- 
mit, in order that the number of individuals may be large enough to indicate 
clearly the resulting adjustment in position and arrangement. The best re- 
sults can be obtained where a small separate area of a formation can be 
modified, e. g., where a small swamp can be drained, or a depression flooded. 
In the case of light, however, it is usually impossible to clear or to shade a 
large area, and the study must be restricted to a relatively small group of 
plants. In regions where lumbering is actively carried on, the consequent 
clearing initiates invaluable experiments over large areas, and this is like- 
wise true of forest plantations. Modification of a large area has decided ad- 
vantages in bringing out the changes in the more prominent structural fea- 
tures, but the causes and the details of the adjustment can be worked out 
much more satisfactorily in a small area. 

359. Denuding. The modification of the habitat by denuding is the sole 
method of initiating succession by experiment. It is consequently of the 
most fundamental importance in investigating aggregation, ecesis, and com- 
petition, as well as the reactions exerted by the invaders of the different 
stages. The possibilities of denuding an entire habitat or an extensive 
area are not great, and the investigator must content himself with denuded 
quadrats, transects, and migration circles, which are small enough to permit 
a critical study of all the factors in succession. It is of course unnecessary 
that the denuding be done by the ecologist himself, provided he is able 
to follow the succession -from the very beginning. Accordingly, it becomes 
possible for him to make the very best use of all those changes wrought 
by man in which the vegetation is destroyed over considerable areas. These 
are essentially natural experiments, and at this point the methods of natural 
and artificial habitats merge. 

The manner of denuding depends in a degree upon the nature of vege- 
tation, but, when time, convenience, and safety are all taken into account, 
the actual removal of the vegetation as indicated under the denuded quadrat 
is by far the most satisfactory. Under certain conditions, flooding or burn- 
ing can be used to advantage, but cases of this kind are infrequent. The 
purpose of the experiment determines the kind of area to be denuded. 



METHOD OF ARTIFICIAL HABITATS 309 

Quadrat, transact, and migration circle are equally valuable for ecesis and 
competition. The quadrat is best adapted to work in a homogeneous area, 
while the transect is suited to a heterogeneous one characterized by zones, 
societies, or communities. It is an advantage to replace the denuded tran- 
sect by a series of denuded quadrats, one for each zone or society, when the 
transect would be too long for convenience. The denuded migration circle 
is invaluable for aggregation and ecesis, since it makes possible the study 
of migration as a distinct function. A series of denuded quadrats, con- 
sisting of one or more in the different stages of a succession, furnishes 
important evidence concerning the development of each stage. By far the 
best method, however, for making a comparative study of the stages of a 
succession is the quadrat sequence. A quadrat is denuded each year, thus 
yielding a complete sequence of miniature stages through the whole course 
of succession. This method is especially valuable when a succession is 
represented by a single example, and there is no opportunity of reconstruct- 
ing it by the comparison of various stages. A quadrat sequence is naturally 
of the greatest value if begun at the time when the first invaders appear. 

360. Modification of the formation by transfer. The study of partial 
and intermittent invasion into an established vegetation is made through 
the transfer of a species or group of species by means of seeding or plant- 
ing. The process differs in no way from that described for experimental 
evolution, except in so far that an endeavor is made to establish a family or 
a community, and not merely a few individuals. Transfer makes possible 
the critical investigation of ecesis under conditions of intense competition, 
as well as the study of aggregation and the origin of plant groups under these 
conditions. Perhaps its greatest value is in the experimental study of al- 
ternation and zonation, especially the former. It is practically impossible 
to determine whether alternation, especially when corresponsive, is due to 
physical or historical causes, i. e., migration and competition, except by 
means of the reciprocal transfer of the species concerned. 

Field cultures for the careful study of ecesis and competition are made bv 
transferring seeds or plants to new or denuded soils. This is practically 
a combination of the methods of modification and transfer. It has a unique 
value in making it possible to initiate artificial successions of almost any 
character that is desired, and to carry them out with the reactions more or 
less under control. This opens up an extremely important field of ex- 
perimental inquiry, which promises to put the study of succession upon a 
much more exact basis. Competition cultures in the field are not essentially 
different from those under control, and they will be considered under the 
next method. 



3io 



THE FORMATION 



METHOD OF CONTROL HABITATS 

361. Competition cultures. Although it is quite possible to carry on 
experiments in invasion and succession in the planthouse, the limited space 
usually available makes this undesirable, except in a few problems where 
control is necessary. Competition cultures, on the other hand, yield better 
results in the planthouse than in the field, since the physical factors and the 
appearance of unwelcome migrants are much more easily controlled. The 
possibilities of the culture method in the study of competition seem inex- 
haustible, and the author has found it necessarv to confine his own investi- 




ng. 83. Simple culture of floating ecads of Ranunculus sceleratus. 

gations to a few of the fundamental problems. In this work, he has 
distinguished several kinds of cultures, based chiefly upon the species con- 
cerned and the arrangement of the individuals. Simple cultures are those 
in which a single species is used. The resulting group is a family, and the 
competition is between like individuals. In such ' cultures, the problem of 
the factors in competition is reduced to its simplest terms. Mixed cultures 
are based upon two or more species, and the problem is correspondingly 
complicated. As a rule, all the seeds have been sown at the same time in 
both simple and mixed cultures, but it has been found desirable to make 
some heterochronous cultures, in which seeds are also sown after the plants 
have appeared. Mixed cultures are distinguished as layered cultures, when 



METHOD OF CONTROL HABITATS 



311 



the species are of very different height. Thus, rosettes have been grown 
with stemmed plants, tall slender forms with low branching ones, erect plants 
with twining and climbing plants, etc. Further evidence as to the nature 
of competition has been sought by means of ccad cultures, and factor cul- 
tures. In the former, plants of different response to water and light are 
grown together under the same conditions, in order to evaluate the part 
played by the nature of the plant. In a factor culture, the area is divided 
into two or more parts which are given different amounts of water or of light, 
in order to determine the influence of slight variations upon the same com- 
petitors. In somewhat similar fashion, an attempt has been made to 




Fig. 84. Mixed culture of Solidago rigida and Onagra biennis. 

ascertain the bearing of biotic factors upon competition. Cultures are 
easily made in which Cxiscuta or parasitic fungi are used to place certain 
species at a disadvantage. Permanent cultures are obtained by allowing the 
plants to ripen and drop their seeds for several generations, just as in 
nature. They are indispensable for determining the final outcome of the 
competition between different species. 

362. Details of culture methods. All competition cultures have been 
made 1 meter square. In other words, they are quadrats, and they are 
treated exactly as denuded quadrats in the field with respect to factor 
readings, charts, and photographs. In the writer's studies, germination 



312 



THE FORMATION 



tests were made of a large number of species, and those selected which 
showed a high per cent of germinability. Since this was the first experi- 
mental study of competition, this test was deemed necessary, but it is quite 
evident that no such selection is made in nature. Consequently, when the 
seeds used are known to be fresh, a germination test is usually superfluous. 
Considerable care was taken also to select species known to be vigorous 
growers, with the result that practically all the species used for experiment 




Fig. 85. Heterochronous culture of Helianthus annuus and Datura 
stramonium. Family culture of Datura^ Verbascum, etc., in the 
foreground. 

were ruderal or subruderal. The species employed, and the kinds of cul- 
tures in which they were grouped were as follows : 

i. Simple culture of Helianthus annuus. The culture plot was divided 
into four equal parts ; 12 seeds were planted in one, 25 in another, 50 in the 
third, and 100 in the fourth. 

2. Mired culture of Helianthus annuus, Panicum virgatum, and Elymus 
canadensis. Twenty-five seeds each of Helianthus and Panicum were 
planted alternately at equal distances in one-half of the plot, while the other 
half was planted similarly with Helianthus and Elymus. 

3. Mixed culture of Solidago rigida and Onagra biennis. Over one-half 
of the plot were scattered 50 seeds of Solidago and 100 of Onagra; over 
the other, 100 and 200 seeds respectively. 



METHOD OF CONTROL HABITATS 313 

4. Layered culture of Laciniaria punctata, Bidens frondosa, Salvia pitcheri, 
Cassia chamae crista and Kuhnia glutinosa. Fifty seeds of each species 
were scattered more or less uniformly over the entire plot. 

5. Layered culture of Silphium laciniatuni, Datura stramonium and Lac- 
tuca ludoviciana. Fifty seeds of Datura and Lactuca, and 25 of Silphium 
were sown uniformly in one-half of the plot. In the other half, 25 holes were 
made at equal intervals, and one seed of each of the three planted in each 
hole. 

6. Ecad culture of Oenothera rhombipetala (xero phytic), Verbascum 
thapsus (meso phytic), and Penthorum sedoides (hydro phytic). One hun- 
dred seeds of Oenothera and 200 each of Verbascum and Penthorum were 
scattered over the plot. 

7. Heterochronous culture of Helianthus annuus and Datura stramon- 
ium. One hundred seeds of Helianthus were scattered over one half, and the 
same number of Datura seeds over the other half of the plot. In both, 
also, 50 seeds were sown in one 4-inch circle, and 25 seeds in a second circle 
at some distance. A month later, 100 seeds of Helianthus were sown in 
the Datura plot, and vice versa. 

8. Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra, 
Datura, Penthorum, Solidago and Verbascum. The plot was divided into 
9 squares and in each were sown 50 seeds of one of these plants. 

9. Community culture. The sowing was made exactly as for the family 
culture, except that 20 seeds of each plant were used. In the middle of 
each square, 5 seeds of a different species were planted. For the Helian- 
thus, Kuhnia, and Panicum groups, Onagra was used; for Bidens, Onagra, 
and Datura, Helianthus was used, and for Penthorum, Solidago, and Ver- 
bascum, Panicum. 

At the time the cultures were started, check plants were sown in pots. 
The most vigorous seedlings were transplanted singly to large pots, and 
grown under conditions of water, light, and soil as similar as possible to 
those of the competition plots. Photographs of check plants and plots were 
made at the proper intervals, and the plots were charted in quadrats to show 
the course of competition. The factors which control competition were 
sought in a critical study of water-content and light values, which is still 
in process. This work has gone far enough to indicate the correctness of the 
view 1 that competition is purely physical in character. It has, moreover, 
been demonstrated that "room" in competition is merely a loose expression 
for the relation between the number of individuals in a given space, and the 
amount of water, light, and temperature available in the same space. 

1 Clements, F. E. The Development and Structure of Vegetation, 166. 1904. 



314 GLOSSARY 



GLOSSARY 



Note: Last terms frequent in compounds are found in their proper place 
alphabetically. The accent is indicated only in those words accented on the 
penult; all others are accented on the antepenult, or recessively. 

abundance, the total number of individuals in an area. 

acospore (Sucr}, point), a plant with awned disseminules. 

acrophyti'um (aKpov, peak), an alpine plant formation. 

act Turn (d/a-?}, rocky coast), a rocky seashore formation; actad, plant of a 

rocky seashore. 
=ad (-0.8775, patronymic suffix), suffix for denoting an ecad. 
adaptable, able to originate ecads; adaptation, the structural response to 

stimuli. 
adjustment, the functional response to stimuli. 

adventicious (adventicius, foreign), invading from distant formations. 
adventive (adventivus, accidental), established temporarily. 
aggregation, the coming together of plants into groups. 
agi'um (dy>7, beach), a beach formation; agad, a beach plant. 
agri'um (dypd?, field), a culture formation; agrad, a cultivated plant. 
aiphyti'um (d«, permanent), an ultimate formation. 
alsi'um (d'Ao-os, grove), a grove formation; alsad, a grove plant. 
alternation, the heterogeneous arrangement of plant groups and formations 

universally present in vegetation. 
amathi um (d/xaflos, sand of the plain), a sandhill or sandplain formation; 

amathad, a sandhill plant. 
ammochthi'um (d/*./*os, sand, ox&i, bank), a sand bank formation; ammoch- 

thad, a sand bank plant. 
ancium (dyKos, mountain glen), a canyon formation; ancad, a canyon plant. 
anemi'um (di/e/Aos, wind), a blowout formation; anemad, a blowout plant; 

anemochore, a plant distributed by wind. 
-anum (locative suffix), a suffix denoting a layer. 
apostrophe (d7rd, away from, arpo4>yj, a turning), the arrangement of the row 

of chloroplasts parallel to the rays of light. 
apparent noon, the time when the sun crosses the meridian, i.e., sun noon as 

distinguished from noon, standard time. 
=ard (apSov, water of the land), combining term for water-content; ardium, 

a succession due to irrigation. 
ardesiacus, slate colored. 



GLOSSARY 315 

-are (locative suffix), suffix denoting a community. 

aspect (asfiectus, appearance), the seasonal impress of a formation, e.g., the 
spring aspect. 

association, the arrangement of individuals in vegetation. 

atmometer (ar/xos, vapor), an instrument for measuring evaporation. 

atropurpureus, dark purple. 

atrovirens, dark green. 

autochore (auros, self), motile plants, or those with motile spores; autoch- 
thonous (x#wv, ground), native. 

avellaneus, drab. 

barrier, a physical or biological obstacle to migration or ecesis. 

bathyphyti'um (fiaQvs, low), a lowland plant formation. 

blastochore (/SAao-r^, growth), a plant distributed by offshoots. 

-bole (^0X77, a throw), combining term for propulsion; bolochore, a plant dis- 
tributed by propulsion. 

broti'um (/3poro's, mortal), a succession caused by man; brotochore, a plant 
distributed by man. 

caeruleus, pale blue. 

caesius, eye-blue. 

camni'um (/ca/xvw, cultivate), a succession due to cultivation. 

carphospore (Kapc^os, scale), a plant with disseminules possessing a scaly or 

Ghaffy pappus. 
carpostrote (Kap7ro?, fruit), a plant migrating by means of fruits. 
centrospore (Kei/rpov, spur), a plant with spiny disseminules. 
chalici'um (xaAi£, gravel), a gravel slide* formation; chalicad, a gravel slide 

plant. 
cheradi'um (xepaSos, a sandbar), a wet sandbar formation cheradad, a wet 

sandbar plant. 
chersi'um (-^epa-os, dry barren waste), a dry waste formation; chersad, plant 

of a dry waste. 
chioni'um (x^v, oVos, snow), a snow formation; chionad, a snow plant; 

chionophyti'um, a niveal plant formation. 
chledi'um (xAr/Sos, rubbish), a ruderal formation; chledad, a ruderal plant. 
chlorenchym (xAoopos, greenish yellow, evx^/xa, infusion), the chlorophyll tissue 

of the leaf. 
-chore (xwpeco, to spread abroad), combining term to denote agent of migration. 
chresard (xpw<-s, use), the available water of the soil, the physiological water- 
content. 
clitochore (kXltos, slope), a plant distributed by gravity. 



3l6 GLOSSARY 

clysi'um (kXvo-ls, a flooding), a succession in a flooded soil. 

=colus (koAos, dwelling in), combining term for habitat forms. 

community, a mixture of the individuals of two or more species, a group of 
families. 

comospore (kc/jlyj, hair) a plant with hairy or silky disseminules. 

competition, the relation between plants occupying the same area, and depend- 
ent upon the same supply of physical factors. 

consocies, that subdivision of a formation controlled by a facies. 

copious, used of species in which the individuals are arranged closely but 
uniformly. 

coryphi'um (Kopvcfirj, peak), an alpine meadow formation; coryphad, an al- 
pine meadow plant. 

creatospore (/cpea?, aro<s, meat), a plant with nut fruits. 

cremni'um (k^/xvos, crag, cliff), a cliff formation; cremnad, a cliff plant. 

creni'um (Kprjvq, spring), a spring formation; crenad, a spring plant. 

crymi'um (Kpv/xos, frost), a polar barren formation; cry mad, a polar plant; 
crymophytic, pertaining to polar plants. 

crystallochore (k/ovo-toAAos, ice), a plant distributed by glaciers. 

cyaneus, azure. 

cyriodoche (ki^io?, regular), a normal succession. 

dendri'um (SeVSpa, fruit trees), an orchard formation; dendrad, an orchard 
plant. 

derived, coming from other formations or regions, not native. 

diphotic (Sc-, two), the two surfaces unequally lighted; diphotophyll, a leaf dif- 
ferentiated into palisade and sponge tissues owing to unequal illumination. 

diplophyll (Si7rAoos, two-fold), an isophotic leaf with water-storage cells in the 
middle. 

disseminule {semen, seed), a seed fruit modified for migration. 

dissophyte (Sio-o-os, double), a plant with xerophytic leaves and stems, and 
mesophytic roots. 

=doche (Soxn, succession), succession. 

drimi'um (SpLfxvs, biting, pungent), an alkaline habitat, and the corresponding 
formation; drimad, a plant of such a formation. 

driodi'um (Spi'os, thicket), a dry thicket formation; driodad, plant of a dry 
thicket. 

dysgeogenous (Svs-, bad, yrj, soil), weathering with difficulty to form soil. 

ecad (oTkos, home), a habitat form due to origin by adaptation; ece'sis (oIktjo-ls, 
act of coming to be at home) , the germination and establishment of in- 
vaders; ecograph, an instrument for measuring a physical factor of a 
habitat; ecotone (tovos, tension), the tension line between two zones, for- 
mations, consocies, etc. 



GLOSSARY 317 

ecballi'um, (eK/?aAA<o, cut down forests), a succession due to lumbering. 

echard (e^w, to withhold), the non-available water of the soil. 

edobole (oUos, swelling), a plant whose seeds are scattered by propulsion 
through turgescence. 

efficient difference, the amount of a physical factor necessary to produce a 
change in the response. 

enauli'um (ei/avAos, hollow channel), a sanddiaw formation; enaulad, a sand- 
draw plant. 

ende'mic (iv, within Srjfxos, district), occurring in a single formation, or natural 
region; ende'mism, the condition of growing in but one natural area. 

epistrophe (ori, towards, arpocfirj, a turning), the arrangement of the row of 
chloroplasts at right angles to the incident light. 

eremi'um (tprjfxos, desert), a desert formation; eremad, a desert plant. 

estival, pertaining to summer. 

=etum (locative suffix), suffix used to denote a consocies. 

eugeogenous (ev-, well, yrj, soil), weathering readily to form soil. 

facies, a dominant species of a formation: a distinct area controlled by it is a 

consocies. 
family, a group of individuals belonging to one species. 
fixity, the condition characterized by little or no response to stimuli. 
fiavovirens, yellow green. 
forewold, equivalent to the German "vorwald," the thicket zone bordering a 

forest. 

-genous (yeW, to produce), producing. 

geotome (yrj, earth, ro/xr}, edge), an instrument for obtaining soil samples. 
gloeospore (yAoios, sticky stuff), a plant with viscid disseminules. 
-graph (ypa<£?7, a writing), combining term for a recording instrument. 
gregarious (gregarius, grouped in herds), used of species in which the indi- 
viduals occur in groups. 

habitat, a definite physical area characterized by a formation; habitat form, 
the impress given the plant by the habitat. 

harmosis (ap/xo<ns, an adapting), response to stimuli, comprising both adjust- 
ment and adaptation. 

hedium (ISos, a sitting, base), a succession in a residuary soil. 

heliad (^\tos, sun), a heliophyte; heliophyll, the leaf of a sun plant; helio- 
phyte, a sun plant; heliophyti'um, a sun plant formation; heliophilous, 
sun-loving. 



3l8 GLOSSARY 

heli'um (e'Ao?, marsh), a marsh formation; helad, a marsh plant; helodi'um 
(IA.C0S77?, marshy), a swampy open woodland formation; helodad, a marsh 
plant; helodrium (Sptbs, thicket), a thicket formation; helodrad, a plant 
of a marshy thicket; helohyli'um (vXrj, forest) a marsh forest formation; 
helohylad, a marsh forest plant. 

hepodoche (eVco, follow), a secondary succession. 

hizometer (7£a), to sink), an instrument for measuring gravitation water. 

holard (o\os, whole), the total water-content of the soil. 

hydrad (v8/>o-, water), a hydrophyte; hydrochore, a plant distributed by water; 
hydroharmose, response to water stimuli; hydrophyll, the leaf of a 
hydrophyte; hydrophyte, a water plant; hydrophyte um, a water plant 
formation; hydrophilous, water-loving; hydrostatic (o-rartKo'?, stand- 
ing), completing the succession under hydrophytic conditions; hydro- 
tropic (rpo7TiKos, turning), applied to successions which become mesophytic. 

hygrome'tric (vypc's, wet), measuring or absorbing water; hygroscopic 
(o-K07re'a), look), measurable only by a hygroscope; able to absorb moisture. 

hyli'um (vXrj, forest), a forest formation; hylad, a forest plant; hylocolum, 
dwelling in a forest; hylodi'um (vA-wS^s, wooded), a dry open woodland 
formation; hylodad, a plant of this formation; hylophyte, a forest plant. 

hypsi'um (ity/os, elevation), a succession caused by elevation. 

=ile (locative affix), suffix denoting a society. 

immobile, without effective devices for migration. 

indigenous (i?idigena, sprung from the land), native. 

insolation, exposure to intense heat and light. 

isabellinus, leather-colored. 

isolation, separation by barriers. 

isopho'tic (to-os, equal), equally illuminated; isophotophyll, a leaf in which 

both halves of the chlorenchym are alike, due to equal illumination. 
=ium (-etov, locative affix), suffix denoting a formation. 

labile, plastic, easily modified. 

lauri'um (Xavpa, drain), a drain formation; laurad, a drain plant. 

limni'um (Xtfivrj, lake), a lake formation; limnad, a lake plant; limnodium 

(Ai/xvwSes, marshy ground), a salt marsh formation; limnodad, a plant of 

a salt marsh. 
lochmi'um (Ao'x^, thicket), a thicket formation; lochmad, a thicket plant. 
lophi'um (Ao<£os, crest, hill), a hill formation; lophad, a hill plant; lopho= 

spore, a plant with plumose disseminules. 

mastigospore (fido-rig, tyos, lash), a plant with ciliate or flagellate disseminules. 
melleus. honey-colored. 



GLOSSARY 319 

meridian, used chiefly as a synonym for apparent noon; also an imaginary 
line of longitude. 

mesad (/xeo-o?, middle), a mesophyte; mesophilous, growing in moist soils; 
mesophyll, the leaf of a mesophyte; mesophyte, a plant of moist soils; 
mesophyti'um, a mesophytic formation; mesosta'tic (orartKcs, stand- 
ing), completing the succession under mesophytic conditions; mesotro'pic 
(Tpo-n-iKos, turning), applied to successions which become mesophytic. 

-meter (fxerpov, measure), combining term for instrument. 

micti'um (jjllktov, mixture), a mixed formation. 

migrant, a plant that is migrating or invading. 

migration (migratio, removal), the movement of plants into new areas; mi- 
gration circle, a circle employed to measure migration. 

mobile, able to be moved, i. e. , modified for migration. 

monochronic (/xc'vos, single, x oovo? > time), arising but once; monogenesis 
(yei/ecri?, origin), the origin of a new form at a single place or time; 
monophyle'sis (<£vAov, race), origin from a single ancestral type; mono- 
to'pic (tottos, place), arising at one place only. 

motile, able to move by growth, by means of cilia, etc. 

mutable, able to produce mutants; mutant, a form arising by mutation; rau= 
tation, the sudden appearance of new forms. 

namati urn (vdfxa, aro», brook), a brook formation; namatad, a brook plant. 
nomi'um (vo/xo's, pasture), a pasture formation; nomad, a pasture plant. 

occupation, possession of the ground by plants. 

oceani'um (cLxeavos, ocean), an ocean formation; oceanad, an ocean plant; 

oceanophyte, an ocean plant; oceanophilous, ocean-dwelling. 
ocheti'um (ox^ros, drain), a succession due to artificial drainage. 
ochroleucus, yellowish white. 

ochthi'um (ox^, bank), a bank formation; ochthad, a bank plant. 
oligope'lic (oAiyos, little, ^77X05, clay), containing little clay; oligopsam'mic 

(i^a/x/xo?, sand), containing little sand. 
olisthi'um (6'Aicr#os, slip), a succession in a landslip. 
ombrometer (S/xfipos, a rainstorm), a rain gauge. 
=on (-wv, locative suffix), suffix used to denote a family. 
oncospore (oyxo?, hook) , a plant with hooked disseminules. 
orgadi'um (opyas, dSos, meadowiand partially wooded), an open woodland 

formation; orgadad, an open woodland plant. 
orophyti'um (opos, mountain), a subalpine plant formation. 
oxodi'um (o^wSt??, sour), a humus marsh formation; oxodad, a plant of a 

humus marsh. 



320 GLOSSARY 

pagi'um (7rayos, rocky hill, glacier), a succession in a glacial soil; pagophy- 
ti'um, a foothill plant formation. 

pediophyti'um (7reSi'ov, plain), an upland plant formation. 

pelagi'um (Tre'Aayos, surface of the sea), a surface sea formation; pelagad, a 
plant of the sea surface. 

pelochthi'um (707X05, mud, oxOrj, bank), a mud bank form; pelogenous, pro- 
ducing clay; pelopsammic (i//a/x/xos, sand), composed of mixed clay and 
sand; pelopsammogenous, producing clay and sand. 

permobile, extremely mobile. 

perquadrat, a quadrat of 16 square meters or more. 

petasospore (7reVao-os, sunshade), a plant with parachute-like disseminules. 

petri'um (irerpa, rock, stone), a rock formation; petrad, a rock plant; 
petrochthi'um (oxOrj, bank), a rock bank formation. 

petrodi'um (TrerpwS^s, abounding in boulders), a boulder field formation; pe- 
trodad, a plant of a boulder field. 

phelli'um (<£e\Aeus, stony ground), a rock field formation; phellad, a rock 
field plant. 

=philous (<£i\os), loving, dwelling in. 

-photic (c/>w?, 4>u)tgs, light), pertaining to light; photoharmose, response to 
light stimuli; photometer, an instrument for measuring light. 

phreti'um (cpprjTos, tank), a tank formation; phretad, a tank plant. 

phyad (<£™j, form of growth), a vegetation form, e. g. , tree, shrub, etc. 

=phyll (cf>v\\ov, leaf), combining term for leaf. 

=phyte (4>vt6v, plant), combining term denoting plant; phyteris (ept?, strife), 
plant competition; =phyti'um ((/>uraov, place covered with plants), com- 
bining term for formation; phytostrote, a species migrating by means of 
the plant body. 

pladobole (7rAa8os, moisture), a plant whose seeds are scattered by propulsion 
due to moisture. 

plasticity, the condition characterized by ready response to stimuli. 

pnoi'um (7^077, blast), a succession in an aeolian soil. 

poi'um (7rda, meadow), meadow formation; poad, a meadow plant; poophyte, 
a meadow plant. 

polyan'thous (-rroXvs, many, avOos, flower), producing many flowers; poly= 
chro'nic (^pcVos, time), arising at two or more times; polyde'mic (Sfj/xos, 
district), occurring in two or more formations or natural regions; poiy= 
genesis (yeveo-ts, origin), the origin of a new form at two or more places 
or times; polyphyle'sis (cf>v\ov, race), the origin of a form, species, of 
genus from two or more ancestral types; polyspermatous (cnripixa, seed), 
producing many seeds in each flower; polyto'pic (roVos, place), arising 
at two or more distinct places. 



GLOSSARY 321 

ponti'um (ttgVtos, deep sea), a deep sea formation. 

potami'um (7rora^o?, river), a river formation; potamad, a river plant. 

potometer (71-oTov, drink), an instrument for measuring absorption. 

prevernal, pertaining to early spring. 

prior, earlier, used of alpine aspects. 

prochosi'um (npcx^o-Ls, a deposition of mud), a succession in an alluvial soil. 

prodophyti'um (7rpoo8os, pioneer), an initial formation. 

protodoche (7t/dujtos, first), a primary succession. 

proximity (proxzmztas, nearness), nearness to the area invaded. 

psamathi'um (i//a/xa#os, sand of the seashore), a strand formation; psamathad, 

a strand plant; psammogenous (^a/x/xos, sand), producing a sandy soil. 
psili'um (if/iXd, land without trees), a prairie formation; psilad, a prairie 

plant. 
psychrometer (j/wxpos, chill), an instrument that measures humidity by means 

of a fall in temperature; psychrograph, a psychrometer that records 

automatically. 
ptenophyti'uin (tttyjvgs, passing), an intermediate formation. 
pterospore (-n-Tepov, wing), a plant with winged disseminules. 
purpureus, purple. 

pycnophyti'um (ttvkvos, thick), a closed formation. 
pyri'um (mrvp, fire), a burn succession. 

quadrat (quadratuvi, a square), a square meter of vegetation marked off for 
counting, mapping, etc.; major, a quadrat of 2-14 square meters. 

reaction, the effect of the formation upon the habitat. 

relict (relictus, left), a species belonging properly to an earlier type of succes- 
sion than the one in which it is found. 

repi'um (peVw, sink), a succession due to subsidence. 

rhoi'um (poos, stream), a creek formation; rhoad, a creek plant. 

rhoptometer ( ponrov, something absorbed), an instrument to measure absorp- 
tion of water by the soil. 

rhyaci'um (pw^, a/<os, mountain torrent), a torrent formation; rhyacad, a 
torrent plant. 

rhysi'um (/Wis, a flowing of fire), a succession due to volcanic action. 

ruber, red. 

saccospore (o-olkkos, sack), a plant with sack-like disseminules. 

sarcospore (o-dp£, aapKos, flesh), a plant with fleshy disseminules. 

sciad (cr/aa, shade), a sciophyte; sciophyll, the leaf of a shade plant; scio- 

phyte, a shade plant; sciophyti'um, a shade plant formation; sciophil- 

ous, shade-loving. 



322 GLOSSARY 

selagraph (o-e'Aas, light), an instrument for recording light values automatically. 
serotinal, late, pertaining to autumn. 

social, used of plants in which the individuals are compactly grouped; ex- 
clusive, excluding individuals of other species; inclusive, permitting the 

entrance of individuals of other species. 
society, a subdivision of the formation, characterized by a principal species. 
sparse, scattered singly. 

spermatostrote (a-n-epfxa, aro?, seed), a plant migrating by means of seeds. 
sphyri'um (a<f>vpov, ankle, talus), a succession in a talus soil. 
spongophyll (o-7royyos, a sponge), a leaf consisting of sponge tissue. 
sporadophyti'um (o-n-opds, aSo?, scattered), an open formation. 
=spore (a-n-opd, seed, fruit), combining term for migration contrivance; sporo- 

strote, a plant migrating by means of spores. 
stability, the condition in which the plant makes little or no response. 
stabilization, the tendency typical of succession, in which the successive 

stages become more stable. 
stasi'um (o-tcutis, a standing), a stagnant pool formation; stasad, a plant of 

stagnant water. 
staurophyll (aravpos, a pale), a leaf consisting of palisade tissue. 
sterrhi'um (orep/oos, barren), a moor formation; sterrhad, a moor plant. 
=strote ((tt/xotos, strewn), combining term for means of migration. 
subcopious, scattered somewhat loosely. 
subgregarious, arranged in loose groups. 
subquadrat, a quadrat of 1-8 decimeters. 
succession, complete and continuous or repeated invasion, in consequence of 

which formations succeed each other. 
symmetry, used of topography when it shows uniform changes; radial, a 

condition in which the different areas are concentric; bilateral, where the 

areas occur in two similar rows. 
syrtidi'um (o-v/ms, iSos, sandbar), a dry sandbar formation; syrtidad, a plant 

of a dry sandbar. 

taphri'um (ra^po?, ditch), a ditch formation; taphrad, a ditch plant. 
telmati'um (re'A/m, aro?, water meads), a wet meadow formation; telmatad, 

a wet meadow plant. 
testaceus, pale brick colored. 

thalassi'um (OdXaaa-a, sea), a sea formation; thalassad, a sea plant. 
thallostrote (0aAAos, shoot), a species migrating by means of offshoots. 
theri'um (Orjp, wild animal), a succession due to animals. 
thermi'um (Qep/xr), hot spring), a hot spring formation; thermad, a hot spring 

plant. 



GLOSSARY 323 

thini'um (0i's, 6iv6s, a dune), a dune formation; thinad, a dune plant. 
tiphi'um (rt^>o?, pool), a pool formation; tiphad, a pond plant. 
tiri'um (Wp w , rub away), a bad land formation; tirad, a bad land plant. 
tonobole (rovos, tension), a plant whose seeds are scattered by projection 

from calyx or involucre. 
transect (transecius, cut through), a cross-section of vegetation. 
trechometer (V/oe^o), to run off), an instrument for measuring run-off. 
tribi'um (Tpt(3(u, wear or rub away) , a succession in an eroded soil. 

umbrinus, umber. 

variable, able to produce variants; variant, a form arising from origin by 
variation; variation, the origin of new forms by the action of selection 
upon minute differences. 

vegetatian form, a characteristic plant form, e. g. , tree, rosette, etc. 

vernal, pertaining to spring. 

vicine {vicinus, neighboring), invading from adjacent formations. 

viridis, green. 

vixgregarious, arranged in small or indistinct groups. 

water=content, the water of the soil or habitat; physiological, the available 
soil water; physical, the total amount of soil water. 

xenodoche (£«/os, strange), an anomalous succession. 

xerad (£17/005, dry), a xerophyte; xerasi'um (^patrta, drought), a succession 
due to drainage or drought; xeriobole (irfpfa, dryness), a plant whose 
seeds are scattered by dehiscence due to dryness; xerohyli'um (vXrj, 
forest), a dry forest formation; xerohylad, a dry forest plant; xerophyll, 
the leaf of a xerophyte; xerophyte, a dry soil plant; xerophyti'um, a 
xerophytic formation; xerophilous, dwelling in a dry habitat; xero- 
poi'um, a heath formation; xeropoad, a heath plant; xerosta'tic (o-toltlkos, 
standing), used of successions which are completed under xerophytic con- 
ditions; xerotro'pic (t^otti/cos, turning), applied to successions which be- 
come xerophytic. 

zonation, that condition in which plant groups or formations appear in belts 

or zones. 
zone, a belt of more or less uniform vegetation. 
zoochore (£wov, animal), a plant distributed by animals. 



3 2 4 BIBLIOGRAPHY 



BIBLIOGRAPHY 

ASCHERSON, P. 

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